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

gtk/gtktreestore.h

@@ -15,13 +15,13 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#ifndef __GTK_TREE_STORE_H__
+#define __GTK_TREE_STORE_H__
+
 #if !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
 #error "Only <gtk/gtk.h> can be included directly."
 #endif
 
-#ifndef __GTK_TREE_STORE_H__
-#define __GTK_TREE_STORE_H__
-
 #include <gdk/gdk.h>
 #include <gtk/gtktreemodel.h>
 #include <gtk/gtktreesortable.h>