The least complicated thing I can think of is
one of these.
How effective it would be depends on the cover for the light (space requirements, frosting of glass blocking occupancy sensor), and what kind of bulb you have in it. Obviously, this solution would also require the light switch to be on all the time, so not ideal, but cheap and easy.
A more advanced method would be to put a door jamb switch on the door you want to activate the light. A lot more foolproof and reliable than an occupancy sensor, but would require additional wiring, and again would have to keep the light switch on, or blank it out and tie line and load inside the switch box.