Fix air cache being leaked in CheckCaches.

src/openttd.cpp

@@ -1432,6 +1432,7 @@ void CheckCaches(bool force_check)
 		free(grf_cache);
 		free(veh_cache);
 		free(gro_cache);
+		free(air_cache);
 		free(tra_cache);
 		free(veh_old);
 	}