Make wxWindow::KillFocus() private and non-virtual in wxX11

This is not an overridden base class member function, contrary to what
the comment stated.

include/wx/x11/window.h

@@ -177,7 +177,6 @@ protected:
         int incW, int incH);
     virtual void DoCaptureMouse();
     virtual void DoReleaseMouse();
-    virtual void KillFocus();
 
 #if wxUSE_TOOLTIPS
     virtual void DoSetToolTip( wxToolTip *tip );
@@ -187,6 +186,8 @@ private:
     // common part of all ctors
     void Init();
 
+    void KillFocus();
+
     wxString m_Label;
 
     wxDECLARE_DYNAMIC_CLASS(wxWindowX11);