fix compilation without WXDEBUG (includes patch 561458)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15701 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/x11/app.h

@@ -74,7 +74,9 @@ public:
     // Processes an X event.
     virtual bool ProcessXEvent(WXEvent* event);
     
+#ifdef __WXDEBUG__
     virtual void OnAssert(const wxChar *file, int line, const wxChar *msg);
+#endif // __WXDEBUG__
     
 protected:
     bool                  m_showOnInit;