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 <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Geert Uytterhoeven <[e-mail]>

arch/m68k/include/asm/string.h

···
 #define __HAVE_ARCH_MEMMOVE
 extern void *memmove(void *, const void *, __kernel_size_t);
 
+extern int memcmp(const void *, const void *, __kernel_size_t);
 #define memcmp(d, s, n) __builtin_memcmp(d, s, n)
 
 #define __HAVE_ARCH_MEMSET