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/x11/app.h

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