Fix some oversights with header guards

Pointed out in https://bugzilla.gnome.org/show_bug.cgi?id=741254

gtk/gtkplacessidebar.h

@@ -23,13 +23,14 @@
  *            Federico Mena Quintero <federico@gnome.org>
  *
  */
-#if !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
-#error "Only <gtk/gtk.h> can be included directly."
-#endif
 
 #ifndef __GTK_PLACES_SIDEBAR_H__
 #define __GTK_PLACES_SIDEBAR_H__
 
+#if !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
+#error "Only <gtk/gtk.h> can be included directly."
+#endif
+
 #include <gtk/gtkwidget.h>
 
 G_BEGIN_DECLS