Codechange: allow certain enumeration to be added

Otherwise C++20 doesn't like it.

src/livery.h

@@ -59,7 +59,7 @@ enum LiveryScheme : byte {
 DECLARE_POSTFIX_INCREMENT(LiveryScheme)
 
 /** List of different livery classes, used only by the livery GUI. */
-enum LiveryClass {
+enum LiveryClass : byte {
 	LC_OTHER,
 	LC_RAIL,
 	LC_ROAD,
@@ -71,6 +71,7 @@ enum LiveryClass {
 	LC_GROUP_AIRCRAFT,
 	LC_END
 };
+DECLARE_ENUM_AS_ADDABLE(LiveryClass)
 
 /** Information about a particular livery. */
 struct Livery {