Missing required aircraft.cfg value in the SDK

according to the current SDK the general section for an airplane required
atc_type/model and all the Icao_* entries. With only these values an airplane
will not spawn as AI! Checking the ground vehicle general section there is
still the requirment for the category entry. Adding this back int he
aircraft.cfg spawns the aircraft. Please update the SDK so the aicraft.cfg
(airplane) general section gets the category as a required value.