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

include/wx/unix/glx11.h

@@ -146,10 +146,7 @@ public:
     // This method is not currently used by the library itself, but remains for
     // backwards compatibility and also because wxGTK has it we could start
     // using it for the same purpose in wxX11 too some day.
-    virtual void *GetXVisualInfo()
-#ifdef __WXGTK20__
-                                    wxOVERRIDE
-#endif
+    virtual void* GetXVisualInfo() wxOVERRIDE
     {
         return wxGLCanvasX11::GetDefaultXVisualInfo();
     }

include/wx/x11/app.h

@@ -84,6 +84,7 @@ public:
         // with multiple display
         return m_visualInfo;
     }
+    virtual void* GetXVisualInfo() { return NULL; }
 
 public:
     static long           sm_lastMessageTime;