CueEntry must inherit from sigc::trackable

It connects to the ParameterChanged (sigc++) signal, and to ensure automated
disconnection on object death, sigc::trackable is required in the
inheritance tree

gtk2_ardour/cuebox_ui.h

@@ -41,7 +41,7 @@ namespace ArdourCanvas
 	class Circle;
 }
 
-class CueEntry : public ArdourCanvas::Rectangle
+class CueEntry : public ArdourCanvas::Rectangle, public sigc::trackable
 {
 public:
 	CueEntry (ArdourCanvas::Item* item, uint64_t cue_index);