compatibility with new monad-control

monad-control-1.0.* uses an associated type synonym rather than an associated data type. This caused the error:

     Language/Haskell/GhcMod/Monad.hs:370:5:
         Wrong category of family instance; declaration was for a type synonym
         In the newtype instance declaration for ‘StM’
         In the instance declaration for ‘MonadBaseControl IO (GhcModT m)’

This could be solved by putting an upper  bound on `monad-control`, but that seemed silly; on the other hand, maybe a conditional is needed, though it looked like the newer monad-control doesn't itself have any dependencies that the older ones didn't.