Fix mirage-conduit ocamlfind dependencies

[samoht]

Otherwise, we get:

File "_none_", line 1:
Error: No implementations provided for the following modules:
         Tls_mirage referenced from /Users/thomas/.opam/vpnkit/lib/mirage-conduit/conduit_mirage.cmxa(Conduit_mirage)

when linking

[samoht]

/cc @avsm with this patch I was able to update ocaml-git to latest conduit/cohttp.

[avsm]

Bah, we need a test case for this as well here -- is there a simple linkage test that would find this in the future, without pulling in the whole mirage configure dance?

[samoht]

@avsm: yea, just need to build some tests ;p I can add a very simple one, at least to check that linking is working fine.

[samoht]

Simple linking test pushed