Ppxlib's pprintast eats up `pvb_constraint` when the value is a function

[NathanReb]

An identity ppxlib driver would turn:

let f : int -> int = fun x -> x

into:

let f x = x

if used in source mode (i.e. the AST is correct, only the source printer is at fault).

This seems to have been fixed on trunk, maybe even in 5.4. If these part of the AST have not been touched since then, we might be able to simply update the relevant binding function in our copy of Pprintast.