Codechange: allow certain enumeration to be added

Otherwise C++20 doesn't like it.

src/vehicle_gui.h

@@ -22,12 +22,13 @@
 void ShowVehicleRefitWindow(const Vehicle *v, VehicleOrderID order, Window *parent, bool auto_refit = false);
 
 /** The tabs in the train details window */
-enum TrainDetailsWindowTabs {
+enum TrainDetailsWindowTabs : byte {
 	TDW_TAB_CARGO = 0, ///< Tab with cargo carried by the vehicles
 	TDW_TAB_INFO,      ///< Tab with name and value of the vehicles
 	TDW_TAB_CAPACITY,  ///< Tab with cargo capacity of the vehicles
 	TDW_TAB_TOTALS,    ///< Tab with sum of total cargo transported
 };
+DECLARE_ENUM_AS_ADDABLE(TrainDetailsWindowTabs)
 
 /** Special values for vehicle-related windows for the data parameter of #InvalidateWindowData. */
 enum VehicleInvalidateWindowData {