Fix message boxes with wxYES_NO|wxCANCEL style in wxMSW

Windows CE code removal in 8282c1be0f (see
https://github.com/wxWidgets/wxWidgets/pull/81) broke message boxes showing
"Yes", "No" and "Cancel" buttons at once, add accidentally removed code back.

Closes https://github.com/wxWidgets/wxWidgets/pull/139

src/msw/msgdlg.cpp

@@ -460,7 +460,10 @@ int wxMessageDialog::ShowMessageBox()
     const long wxStyle = GetMessageDialogStyle();
     if ( wxStyle & wxYES_NO )
     {
-         msStyle = MB_YESNO;
+        if ( wxStyle & wxCANCEL )
+            msStyle = MB_YESNOCANCEL;
+        else
+            msStyle = MB_YESNO;
 
         if ( wxStyle & wxNO_DEFAULT )
             msStyle |= MB_DEFBUTTON2;