kbuild: Fix missing system calls check on mips.

Commit 5f7efb4c

 (Kbuild: append missing-syscalls to the default target
list) broke MIPS build.

Reported-tested-and-acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>