Blackfin: fix strncmp.o build error

Fix some more fallout of the string changes:

  CC      arch/blackfin/lib/strncmp.o
In file included from include/linux/bitmap.h:9,
                 from include/linux/nodemask.h:90,
                 from include/linux/mmzone.h:17,
                 from include/linux/gfp.h:5,
                 from include/linux/kmod.h:23,
                 from include/linux/module.h:14,
                 from arch/blackfin/lib/strncmp.c:14:
include/linux/string.h: In function ‘strstarts’:
include/linux/string.h:132: error: implicit declaration of function ‘strncmp’
make[1]: *** [arch/blackfin/lib/strncmp.o] Error 1

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Rusty Russell <rusty@rustcorp.com.au>