Fix build problem with "override" on wxApp::GetXVisualInfo()

This method doesn't override any virtual method in the base class, so
wxOVERRIDE can't be used here.

In fact, this method doesn't seem to be used at all, but keep it for
compatibility and in case we want to use it later, as wxGTK does.

include/wx/unix/glx11.h

@@ -143,8 +143,10 @@ public:
         return wxGLCanvasX11::InitDefaultVisualInfo(attribList);
     }
 
-    // and implement this wxGTK::wxApp method too
-    virtual void *GetXVisualInfo() wxOVERRIDE
+    // 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()
     {
         return wxGLCanvasX11::GetDefaultXVisualInfo();
     }