Fix: NewGRF house class mappings were not reset between games. (#11279)

(cherry picked from commit 6643c010bd)

src/newgrf_house.h

@@ -134,6 +134,7 @@ struct HouseClassMapping {
 	uint8  class_id;  ///< The class id within the grf file
 };
 
+void ResetHouseClassIDs();
 HouseClassID AllocateHouseClassID(byte grf_class_id, uint32 grfid);
 
 void InitializeBuildingCounts();