Bicycle horn does not need electricity

Again my bad, sorry. Forgot that bicycles and some-such are also vehicles...
Uh, `return`, since you're checking for horns that are powered and there's `no_power`?

Might want to add_msg something like "The horn doesn't sound...", just so the player knows.  Using the menu still says "You honk the horn!", it just doesn't make any noise with these PRs and no battery power.