sound/oss/ac97_codec.c: restore MODULE_LICENSE

I accidentally removed the module license from sound/oss/ac97_codec.c in
commit 83bad1d7

 ("scheduled OSS driver
removal")

Spotted by Roland <devzero@web.de>.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>