Fix clang -Winconsistent-missing-override in wxGLApp

Use "override" for wxGLApp::GetXVisualInfo(), but only for wxGTK2 where
it really overrides it, unlike in wxX11.

Closes #17995.

include/wx/unix/glx11.h

@@ -147,6 +147,9 @@ public:
     // 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
     {
         return wxGLCanvasX11::GetDefaultXVisualInfo();
     }