m68k: Add memcmp() declaration

There is a global definition of memcmp() that gets built on m68k but is
never used and causes a warning because of the missing prototype:

    lib/string.c:671:15: error: no previous prototype for 'memcmp' [-Werror=missing-prototypes]

Add the corresponding declaration to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230809211057.60514-2-arnd@kernel.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>