Added IG_PRIVATE_AIRSHIP (#3378)

* Item group for Private Airship tickets

src/map/script_constants.hpp

@@ -4977,6 +4977,7 @@
 	export_constant(IG_SPECIAL_CHRISTMAS_BOX);
 	export_constant(IG_SANTA_GIFT);
 	export_constant(IG_PRIZEOFHERO);
+	export_constant(IG_PRIVATE_AIRSHIP);
 
 	/* unit stop walking */
 	export_constant(USW_NONE);