Added stub implementation for 'Fall'

In Morrowind's engine, removing the surface the object is standing on will not wake it up from it's gravity-less state. The purpose of the Fall function is to 'activate' gravity on a sleeping physics object.

There is no use for this function in openmw because our physics controller constantly applies gravity anyway.

The stub implementation makes the Tarhiel script work (minus the fact he doesn't die from falling damage).