seccomp, s390: fix build for syscall type change

A recent patch landed in the security tree [1] that changed the type of the
seccomp syscall. Unfortunately, I didn't quite get every instance of the
forward declarations, and thus there is a build failure. Here's the last
one that I could find, for s390. It should go through the security tree,
although hopefully some s390 people can check and make sure it looks
reasonable?

The only oddity is the trailing semicolon; some lines around this patch
have it, and some lines don't. I've left this one as-is.

[1]: https://lore.kernel.org/lkml/20181212231630.GA31584@beast/T/#u

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
Signed-off-by: Kees Cook <keescook@chromium.org>