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/x11/gdkx11property.h

@@ -22,13 +22,13 @@
  * GTK+ at ftp://ftp.gtk.org/pub/gtk/. 
  */
 
+#ifndef __GDK_X11_PROPERTY_H__
+#define __GDK_X11_PROPERTY_H__
+
 #if !defined (__GDKX_H_INSIDE__) && !defined (GDK_COMPILATION)
 #error "Only <gdk/gdkx.h> can be included directly."
 #endif
 
-#ifndef __GDK_X11_PROPERTY_H__
-#define __GDK_X11_PROPERTY_H__
-
 #include <gdk/gdk.h>
 
 #include <X11/Xlib.h>