timingsafe_memcmp: Fix meson build

Include dix-config.h first to pick up _GNU_SOURCE so we get the
definition for sigset_t.

Reported-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Adam Jackson <ajax@redhat.com>

os/timingsafe_memcmp.c

@@ -14,10 +14,10 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <dix-config.h>
 #include <limits.h>
 #include <string.h>
 #include <X11/Xfuncproto.h>
-#include <dix-config.h>
 #include "os.h"
 
 int