I'd first clean out all of that corrosion/dirt out of the visible hole with wd40/wirebrush cos it can obstruct the screw and maybe why it rounded off.
if the allen hole is very rounded, you can try tapping in a slightly larger Torx key to bite & grab into the plastic screw or use a flat-head that fits onto the slot head.
once removed, if the screw head is damaged you could either cut a new flat-head slot on the screw with a dremel, get a spare screw from the scrapyard or dealership (expensive), or replace the throttle body as a last resort.
