test: block prctl in seccomp_filter

restore_creds uses prctl, so if we block this call in the seccomp filter
test, it causes things to fail (hang actually, seems we have some unhandled
error path here).
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/seccomp_filter.c

@@ -97,7 +97,7 @@ int main(int argc, char ** argv)
 		if (filter_syscall(__NR_ptrace) < 0)
 			_exit(1);
 
-		if (filter_syscall(__NR_getpid) < 0)
+		if (filter_syscall(__NR_prctl) < 0)
 			_exit(1);
 
 		setuid(1000);