include seems to leak val declarations

[mtzguido]

A.fst

module A

include B

let x = 2

B.fsti:

module B

val x : int

Checking A gives:

* Error 162 at A.fst(5,0-5,9):
  - Invalid qualifiers for declaration `let x`
  - Definitions cannot be marked `assume`.

Using an open works fine.