fix OSX compilation (flat namespace)

reference to 'Rect' is ambiguous
..../CarbonCore.framework/Headers/MacTypes.h defines
typedef struct Rect

gtk2_ardour/note.cc

@@ -98,7 +98,7 @@ Note::hide ()
 }
 
 void
-Note::set (Rect rect)
+Note::set (ArdourCanvas::Rect rect)
 {
 	_rectangle->set (rect);
 }