reduce some include dependencies

rtengine/rtthumbnail.h

@@ -18,8 +18,6 @@
  */
 #pragma once
 
-#include <glibmm/ustring.h>
-
 #include <lcms2.h>
 
 #include "image16.h"
@@ -30,6 +28,13 @@
 
 #include "../rtgui/threadutils.h"
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {