set both min and desired size at the same time

otherwise under at least gtk2 the "missing file" dialog from
./soffice no-thing-at-all
will be oversized

Change-Id: I3c0c9a0a8ec0fcd37fb8830940a9d4909e6b5a1b

vcl/source/window/btndlg.cxx

@@ -205,6 +205,7 @@ void ButtonDialog::ImplPosControls()
             nY += maCtrlSize.Height()+IMPL_SEP_BUTTON_Y;
     }
 
+    SetOutputSizePixel(aDlgSize);
     SetMinOutputSizePixel(aDlgSize);
 
     mbFormat = false;