Missing version constraint in opam

[djs55]

See

# File "http-svr/server_io.ml", line 31, characters 6-23:
# Error: Unbound value Debug.name_thread
# Command exited with code 2.
### stderr ###

https://travis-ci.org/mirage/ocaml-tar/jobs/63484920

[gaborigloi]

It looks like we should depend on xapi-idl in the opam file. This means the opam file is wrong here, in xs-opam, and in the main opam repo.

[mseri]

But it has been building fine in all those 😕

[gaborigloi]

It's probably a transitive dependency.