Avoid including X11 headers from <wx/glcanvas.h>

It's generally wxWidgets policy not to include platform-specific headers from our own
to avoid namespace pollution issues, in this case with names like "None" and "Window".

include/wx/gtk/glcanvas.h

@@ -63,7 +63,7 @@ public:
     // implement wxGLCanvasX11 methods
     // --------------------------------
 
-    virtual Window GetXWindow() const wxOVERRIDE;
+    virtual unsigned long GetXWindow() const wxOVERRIDE;
 
 
     // deprecated methods