Fixes ability to build over/under fire doors. (#7128)

# About the pull request

You can now build things on tiles that have fire doors again.

# Explain why it's good for the game

Unintentional that you can't rebuild the windows/doors etc. under a fire
door.


# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl:
fix: You can build on tiles with fire doors again.
/:cl:

code/game/objects/items/stacks/stack.dm

@@ -199,7 +199,7 @@ Also change the icon to reflect the amount of sheets, if possible.*/
 
 			if(R.one_per_turf != ONE_TYPE_PER_BORDER) //all barricade-esque structures utilize this define and have their own check for object density. checking twice is unneeded.
 				for(var/obj/object in usr.loc)
-					if(object.density || istype(object, /obj/structure/machinery/door))
+					if(object.density || istype(object, /obj/structure/machinery/door/airlock))
 						to_chat(usr, SPAN_WARNING("[object] is blocking you from constructing \the [R.title]!"))
 						return