canvas: use fwd decl instead of header inclusion for CairoWidget

libs/canvas/canvas/widget.h

@@ -24,7 +24,7 @@
 #include "canvas/visibility.h"
 #include "canvas/item.h"
 
-#include "gtkmm2ext/cairo_widget.h"
+class CairoWidget; /* should really be in Gtkmm2ext namespace */
 
 namespace ArdourCanvas
 {

libs/canvas/widget.cc

@@ -26,6 +26,8 @@
 #include "canvas/widget.h"
 #include "canvas/debug.h"
 
+#include "gtkmm2ext/cairo_widget.h"
+
 using namespace std;
 using namespace ArdourCanvas;