kernel/sys_ni.c: add dummy sys_ni_syscall() prototype

kernel/sys_ni.c can't #include <linux/syscalls.h> due to cond_syscall(),
but let's tell gcc to not warn with -Wmissing-prototypes.

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