lwt-unix: add missing dependency on lwt.unix
lwt-unix: add missing dependency on lwt.unix

this follows on c46f6e91c175454018109a316b990b68e5514d1b
causes build failure in cohttp-lwt-unix otherwise

samoht/ocaml-conduit,avsm/ocaml-conduit,samoht/ocaml-conduit,mirage/ocaml-conduit,djs55/ocaml-conduit,djs55/ocaml-conduit