Fix wxMDIChildFrame screen position in TDI-based implementations

This notably fixes the problem with wxAuiMDIChildFrame::GetScreenRect()
returning the wrong window position since the changes of
c1bcf16eb9.

Closes #18423.

include/wx/mdi.h

@@ -321,6 +321,11 @@ protected:
         wxWindow::DoMoveWindow(x, y, width, height);
     }
 
+    virtual void DoGetScreenPosition(int *x, int *y) const wxOVERRIDE
+    {
+        wxWindow::DoGetScreenPosition(x, y);
+    }
+
     // no size hints
     virtual void DoSetSizeHints(int WXUNUSED(minW), int WXUNUSED(minH),
                                 int WXUNUSED(maxW), int WXUNUSED(maxH),