zdtm: Fix xids00 test compilation on older distrol

The getsid/getpig require _GNU_SOURCE on some libc-s.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/xids00.c

+#define _GNU_SOURCE
+#include <unistd.h>
 #include <errno.h>
 #include <stdlib.h>
 #include <string.h>
-#include <unistd.h>
 #include <signal.h>
 #include <sys/wait.h>