kbuild: fix -Wimplicit-function-declaration in license_is_gpl_compatible

Add missing <linux/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.

[akpm@linux-foundation.org: remember "linux/"]
Link: https://lkml.kernel.org/r/20221116182634.2823136-1-sam@gentoo.org
Signed-off-by: Sam James <sam@gentoo.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/license.h

@@ -2,6 +2,8 @@
 #ifndef __LICENSE_H
 #define __LICENSE_H
 
+#include <linux/string.h>
+
 static inline int license_is_gpl_compatible(const char *license)
 {
 	return (strcmp(license, "GPL") == 0