Fix inter-header includes

Since we are going to install these headers soon, we need
to make their mutual includes work in the installed location
as well. Also, avoid including individual gtk headers, to
avoid trouble with single-include guards.

gtk/a11y/gtkwidgetaccessibleprivate.h

@@ -18,7 +18,7 @@
 #ifndef __GTK_WIDGET_ACCESSIBLE_PRIVATE_H__
 #define __GTK_WIDGET_ACCESSIBLE_PRIVATE_H__
 
-#include "gtkwidgetaccessible.h"
+#include <gtk/a11y/gtkwidgetaccessible.h>
 
 G_BEGIN_DECLS