Added roof on airport ATC tower

[PatrikLundell]

Summary

None

Purpose of change

Ensure the private airport doesn't rely on roof adding magic, but rather defines all its roofs explicitly.

Describe the solution

Define another Z level with a roof for the tower and hook it into the infrastructure.

Describe alternatives you've considered

Testing

• Hack map.cpp operation map::add_roofs to return immediately to disable the roof adding magic.
• Debug spawn the airport special.
• Teleport to it and climb the tower.
• Teleport up onto the roof.
• Enjoy the view (and verify the roof does exist and the buildings below have their roofs too.
• Restore the code.

Additional context

This is part of a long term effort to try to get rid of the usage of magic to create roofs (as well as getting rid of said magic creating roofs where there those roofs should be gone/nonexistent). Only trees should be allowed to be subjected to magic (unless a good way not to rely on magic is found).
The abandoned textile mill looks quite reasonable without magic roofs, by the way.

It can also be noted that the ATC tower can't be constructed by players, as the construction rules don't allow for any overhangs. Since these things exist in real life, the construction rules ought to change eventually.