Avoid ifdef-ed override specification

Instead, add the base class functions so the derived class always
overrides. If/when wxX11 and wxMotif are removed there won't be an
ifdef that no one ever notices isn't needed anymore.

include/wx/motif/app.h

@@ -80,6 +80,7 @@ public:
     virtual void   HandlePropertyChange(WXEvent *event);
 
     wxXVisualInfo* GetVisualInfo(WXDisplay* display);
+    virtual void* GetXVisualInfo() { return NULL; }
 
 private:
     // Motif-specific