lib/libcrc32c.c: fix build warning

Fix the following build warning:

  lib/libcrc32c.c:42:5: warning: no previous prototype for "crc32c" [-Wmissing-prototypes]
   u32 crc32c(u32 crc, const void *address, unsigned int length)
       ^

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>