Let-in-annotation may introduce "unused variables"

For instance: 
```
let () = 
  let[@landmark] rec f () = ()
  and g () = f () in 
  g ()
```
will produce unused variable "f". 
This is a bug in the ppx "translation".

On the other hand, the warning will disappear with:
```
let f = ()
let () = 
  let[@landmark] rec f () = ()
  and g () = f () in 
  g ()
````
this seems to be a bug in the ocaml compiler.
