Add missing 'not' (#28)

otherwise large doors won't swing open anymore

gate_functions.lua

@@ -291,7 +291,7 @@ local get_door_layout = function(pos, facedir, player)
 				if not vector.equals(door_node.pos, origin) then
 					-- There's no obstruction if the node is literally located along the rotation axis
 					local newpos = rotate_pos_displaced(door_node.pos, origin, axis, direction)
-					if get_buildable_to(newpos) then
+					if not get_buildable_to(newpos) then
 						-- check if the destination node is free.
 						door.swings[direction] = false
 						break