Plenty of good suggestions, especially the triangular butt of a file or what not. This is the cheap version of an Easy Out tool.
If you're only going to do this once, then those methods will work as long as you can drill a decent hole in the screw shaft.
Just for referebce, the photo is my own easy outs, and you do the same method as posted above. Drill a hole suitable for the size of the Easy Out. You'll need a smaller one than i have here.
Tap the Easy Out bit down into the drilled hole and wind it it back out, either with the proper tool, like those that come with a tap and die set, or some Mole Grips.