fix require_resource in [resource]

previously https://github.com/wesnoth/wesnoth/blob/918cdfcb36054e04c6663e903c731943e823ddc2/src/saved_game.cpp#L293 would fail for [resource] because cfg["addon_id"] was empty.

(cherry-picked from commit 7ef9f9a)

src/game_config_manager.cpp

@@ -440,7 +440,7 @@ void game_config_manager::load_addons_cfg()
 			cache_.get_config(addon.main_cfg, umc_cfg);
 
 			// Annotate "era", "modification", and scenario tags with addon_id info
-			static const std::set<std::string> tags_with_addon_id { "era", "modification", "multiplayer", "scenario", "campaign" };
+			static const std::set<std::string> tags_with_addon_id { "era", "modification", "resource", "multiplayer", "scenario", "campaign" };
 
 			for(auto child : umc_cfg.all_children_range()) {
 				if(tags_with_addon_id.count(child.key) > 0) {