revert "kbuild: fix -Wimplicit-function-declaration in license_is_gpl_compatible"

It causes build failures with unusual CC/HOSTCC combinations.

Quoting
https://lkml.kernel.org/r/A222B1E6-69B8-4085-AD1B-27BDB72CA971@goldelico.com:

HOSTCC scripts/mod/modpost.o - due to target missing
In file included from include/linux/string.h:5,
from scripts/mod/../../include/linux/license.h:5,
from scripts/mod/modpost.c:24:
include/linux/compiler.h:246:10: fatal error: asm/rwonce.h: No such file or directory
246 | #include <asm/rwonce.h>
| ^~~~~~~~~~~~~~
compilation terminated.

...

The problem is that HOSTCC is not necessarily the same compiler or even
architecture as CC and pulling in <linux/compiler.h> or <asm/rwonce.h>
files indirectly isn't a good idea then.

My toolchain is providing HOSTCC = gcc (MacPorts) and CC = arm-linux-gnueabihf
(built from gcc source) and all running on Darwin.

If I change the include to <string.h> I can then "HOSTCC scripts/mod/modpost.c"
but then it fails for "CC kernel/module/main.c" not finding <string.h>:

CC kernel/module/main.o - due to target missing
In file included from kernel/module/main.c:43:0:
./include/linux/license.h:5:20: fatal error: string.h: No such file or directory
#include <string.h>
^
compilation terminated.

Reported-by: "H. Nikolaus Schaller" <hns@goldelico.com>
Cc: Sam James <sam@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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>

export,module: add SPDX GPL-2.0 license identifier to headers with no license

Commit b24413180f56 ("License cleanup: add SPDX GPL-2.0 license
identifier to files with no license") took care of a lot of files
without any license information.

These headers were not processed by the tool perhaps because they
contain "GPL" in the code.

I do not see any license boilerplate in them, so they fall back to
GPL version 2 only, which is the project default.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20191018045053.8424-1-yamada.masahiro@socionext.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kbuild: check license compatibility when building modules

Modules that uses GPL symbols can no longer be build with kbuild,
the build will fail during the modpost step.
When a GPL-incompatible module uses a EXPORT_SYMBOL_GPL_FUTURE symbol
then warn during modpost so author are actually notified.

The actual license compatibility check is shared with the kernel
to make sure it is in sync.

Patch originally from: Andreas Gruenbacher <agruen@suse.de> and
Ram Pai <linuxram@us.ibm.com>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>