I've had that kind of thing happen a number of times. I swear I try to avoid debris in the road, both for my own safety and to prevent projectiles. But you can't miss them every time. I think the legal standard on this would be that a rider should make reasonable efforts to avoid launching stones, but if it happens, the liability ultimately falls on the city/county/state government (depending on the road), because they are responsible for maintaining the roads. If a car were to fling a stone at you by mistake, you could not really hold the driver liable,* so what's good for the goose...

* This applies if the stone is on the road surface. It's different if something flies out of a truck bed, or if a construction vehicle picks up debris from a job site, fails to get cleaned, and then starts flinging stuff from its tires or chassis.