m68k: assign syscall number for seccomp

Most architectures have assigned a numbers for the seccomp syscall
even when they do not implement it.

m68k is an exception here, so for consistency lets add the number.
Unless CONFIG_SECCOMP is implemented, the system call just
returns -ENOSYS.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>