opam: depend on irmin-unix, to get irmin.unix
opam: depend on irmin-unix, to get irmin.unix

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

djs55/ocaml-xenstore-server,mirage/ocaml-xenstore-server,mirage/ocaml-xenstore-server