Move single-include guards inside include guards

gcc has optimizations for include guards that only work
if they are outermost in the the header.
https://bugzilla.gnome.org/show_bug.cgi?id=689810

gdk/gdkscreen.h

@@ -19,13 +19,13 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#ifndef __GDK_SCREEN_H__
+#define __GDK_SCREEN_H__
+
 #if !defined (__GDK_H_INSIDE__) && !defined (GDK_COMPILATION)
 #error "Only <gdk/gdk.h> can be included directly."
 #endif
 
-#ifndef __GDK_SCREEN_H__
-#define __GDK_SCREEN_H__
-
 #include <cairo.h>
 #include <gdk/gdkversionmacros.h>
 #include <gdk/gdktypes.h>