Here's three options for you.
1. Drill a small hole, remove with small screw extractor. You must be careful doing this. If you mess up the threads at all the new screw will leak.
2. Replace the bonnet of the valve or replace the whole valve. You could buy a new valve and swap out all the parts.
3. Do nothing. The valve will work just fine with that screw broke off. You just wont be able to turn it on manually with the bleeder screw. Try unscrewing the solenoid slowly until the valve comes on.