Add missing dependency on Unix #1458 
Add missing dependency on Unix #1458 

from dra27/unix-dep

Silences the alert proposed in ocaml/ocaml#11198,
rgrinberg/merlin,ocaml/merlin,rgrinberg/merlin,rgrinberg/merlin,rgrinberg/merlin,ocaml/merlin,ocaml/merlin,ocaml/merlin