seccomp: Work-around GCC 4.x bug in gnu99 mode

seccomp: Work-around GCC 4.x bug in gnu99 mode

We'd like to compile QEMU with -std=gnu99, but GCC 4.8 currently
fails to compile qemu-seccomp.c in this mode:

qemu-seccomp.c:45:1: error: initializer element is not constant
 };
 ^
qemu-seccomp.c:45:1: error: (near initialization for ‘sched_setscheduler_arg[0]’)

This is due to a compiler bug which has just been fixed in GCC 5.0:

 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63567

Since we still want to support GCC 4.8 for a while and also want to use
gnu99 mode, work-around the issue by expanding the macro manually.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>