Codechange: allow certain enumeration to be added

Otherwise C++20 doesn't like it.

src/vehicle_gui.cpp

@@ -2587,7 +2587,7 @@ struct VehicleDetailsWindow : Window {
 		const Vehicle *v = Vehicle::Get(this->window_number);
 
 		if (v->type == VEH_TRAIN) {
-			this->LowerWidget(this->tab + WID_VD_DETAILS_CARGO_CARRIED);
+			this->LowerWidget(WID_VD_DETAILS_CARGO_CARRIED + this->tab);
 			this->vscroll->SetCount(GetTrainDetailsWndVScroll(v->index, this->tab));
 		}