Fix erroneous "unknown attribute discarded" warnings for new [unit] keys

src/units/unit.cpp

@@ -103,6 +103,7 @@ namespace
 		"parent_type", "description", "usage", "halo", "ellipse",
 		"upkeep", "random_traits", "generate_name",
 		"profile", "small_profile",
+		"passable", "overwrite", "location_id",
 		// Useless attributes created when saving units to WML:
 		"flag_rgb", "language_name", "image", "image_icon"
 	};