replace m_insertCallback with a virtual function, contrary to the old comments a virtual works just fine

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55354 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/filedlg.h

@@ -62,6 +62,7 @@ protected:
 private:
     void OnFakeOk( wxCommandEvent &event );
     void OnSize(wxSizeEvent&);
+    virtual void AddChildGTK(wxWindowGTK* child);
 
     wxGtkFileChooser    m_fc;