Impassible states shouldn't get extras

[Idhrendur]

Right now there's no check preventing them from getting factories, airbases, infrastructure bonuses, etc. This should not be.

(If working on this, do so from the NSB branch until it's fully merged, else it'll cause a merge conflict)

[gawquon]

This would seem to be a non-issue currently as the impassable states are all categorized as wasteland, which prevents most buildings from being built there. Forts/Air Bases/Infrastructure can all be built there but that is the case in vanilla as well.
The only potential oddity is that there can be resources in these states.

state={
	id=296
	name="STATE_296"
	resources={
		steel=1.000
	}
	impassable = yes

	history={
		owner = GER
		add_core_of = GER
	}

	provinces={
		1057 1140 1994 4191 7985 10104 10754 10785 12866 
	}
	manpower=515711
	buildings_max_level_factor=1.000
	state_category=wasteland

	local_supplies=10.0
}