Fix cast in wxWindowMSW code in wxUniv build

Cast "this" to wxWindowMSW, which is always its correct type, even in
wxUniv builds.

Closes #18670.

src/msw/window.cpp

@@ -4808,7 +4808,7 @@ wxSize wxWindowMSW::GetDPI() const
 
     if ( hwnd == NULL )
     {
-        const wxWindow* topWin = wxGetTopLevelParent(const_cast<wxWindow*>(this));
+        const wxWindow* topWin = wxGetTopLevelParent(const_cast<wxWindowMSW*>(this));
         if ( topWin )
         {
             hwnd = GetHwndOf(topWin);