Take a small steel cold chisel. Hold it quite perpendiculary to the screw axle and apply it tangential to the border of the screw head with a small angle in order to be able to unscrew when gently tapping with a small hammer. The chisel penetrates a little on the border of the screw head and turns it. Don't shear the screw head.
I don't know if it is very clear, but I succeeded to do it by this way.
