kbuild: Fix -Wimplicit-function-declaration in license_is_gpl_compatible

  Add missing <string.h> include for strcmp.

Clang 16 makes -Wimplicit-function-declaration an error by default. Unfortunately,
out of tree modules may use this in configure scripts, which means failure
might cause silent miscompilation or misconfiguration.

For more information, see LWN.net [0] or LLVM's Discourse [1], gentoo-dev@ [2],
or the (new) c-std-porting mailing list [3].

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
[2] https://archives.gentoo.org/gentoo-dev/message/dd9f2d3082b8b6f8dfbccb0639e6e240
[3] hosted at lists.linux.dev.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: trivial@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Sam James <sam@gentoo.org>
---
 include/linux/license.h | 2 ++
 1 file changed, 2 insertions(+)
  

> On 16 Nov 2022, at 18:26, Sam James <sam@gentoo.org> wrote:
> 
> Add missing <string.h> include for strcmp.
> 
> Clang 16 makes -Wimplicit-function-declaration an error by default. Unfortunately,
> out of tree modules may use this in configure scripts, which means failure
> might cause silent miscompilation or misconfiguration.
> 
> For more information, see LWN.net [0] or LLVM's Discourse [1], gentoo-dev@ [2],
> or the (new) c-std-porting mailing list [3].
> 
> [0] https://lwn.net/Articles/913505/
> [1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
> [2] https://archives.gentoo.org/gentoo-dev/message/dd9f2d3082b8b6f8dfbccb0639e6e240
> [3] hosted at lists.linux.dev.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: trivial@kernel.org
> Cc: stable@vger.kernel.org
> Signed-off-by: Sam James <sam@gentoo.org>
> ---
> include/linux/license.h | 2 ++
> 1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/license.h b/include/linux/license.h
> index 7cce390f120b..1c0f28904ed0 100644
> --- a/include/linux/license.h
> +++ b/include/linux/license.h
> @@ -2,6 +2,8 @@
> #ifndef __LICENSE_H
> #define __LICENSE_H
> 
> +#include <string.h>
> +

Andrew's added this to his tree with the following fixup (duh!):
```
-#include <string.h>
+#include <linux/string.h>
```

Best,
sam
  

Hi Sam,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on akpm-mm/mm-everything]
[also build test ERROR on linus/master v6.1-rc5 next-20221116]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Sam-James/kbuild-Fix-Wimplicit-function-declaration-in-license_is_gpl_compatible/20221117-022718
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20221116182634.2823136-1-sam%40gentoo.org
patch subject: [PATCH] kbuild: Fix -Wimplicit-function-declaration in license_is_gpl_compatible
config: arc-defconfig
compiler: arc-elf-gcc (GCC) 12.1.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/intel-lab-lkp/linux/commit/f9de22a17ac9a7c6290945983b336296c3d4766b
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Sam-James/kbuild-Fix-Wimplicit-function-declaration-in-license_is_gpl_compatible/20221117-022718
        git checkout f9de22a17ac9a7c6290945983b336296c3d4766b
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross W=1 O=build_dir ARCH=arc SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   In file included from kernel/module/main.c:43:
>> include/linux/license.h:5:10: fatal error: string.h: No such file or directory
       5 | #include <string.h>
         |          ^~~~~~~~~~
   compilation terminated.


vim +5 include/linux/license.h

     4	
   > 5	#include <string.h>
     6
  

Hi Sam,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on akpm-mm/mm-everything]
[also build test ERROR on linus/master v6.1-rc5 next-20221116]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Sam-James/kbuild-Fix-Wimplicit-function-declaration-in-license_is_gpl_compatible/20221117-022718
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20221116182634.2823136-1-sam%40gentoo.org
patch subject: [PATCH] kbuild: Fix -Wimplicit-function-declaration in license_is_gpl_compatible
config: mips-randconfig-r024-20221117
compiler: clang version 16.0.0 (https://github.com/llvm/llvm-project bbe6bd724a6335e497c7edaed191d37a828d0390)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install mips cross compiling tool for clang build
        # apt-get install binutils-mipsel-linux-gnu
        # https://github.com/intel-lab-lkp/linux/commit/f9de22a17ac9a7c6290945983b336296c3d4766b
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Sam-James/kbuild-Fix-Wimplicit-function-declaration-in-license_is_gpl_compatible/20221117-022718
        git checkout f9de22a17ac9a7c6290945983b336296c3d4766b
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=mips SHELL=/bin/bash kernel/

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   In file included from kernel/bpf/syscall.c:20:
>> include/linux/license.h:5:10: error: 'string.h' file not found with <angled> include; use "quotes" instead
   #include <string.h>
            ^~~~~~~~~~
            "string.h"
   1 error generated.


vim +5 include/linux/license.h

     4	
   > 5	#include <string.h>
     6