dune: add dependency to unix library

[psafont]

I've been getting this error without this when trying to use the library:

### output ###
# (cd _build/default && /home/paus/.local/share/opam/toolstack-local-no-async/bin/ocamlopt.opt -w -40 -w -39-6@5 -g -o ocaml/xapi-storage-script/main.exe /home/paus/.local/share/opam/toolstack-local-no-async/lib/base/base_internalhash_types/base_internalhash_types.cmxa -I /home/paus/.local/share/opam/toolstack-local-no-async/lib/base/base_internalhash_types /home/paus/.local/share/opam/toolstac[...]
# File "_none_", line 1:
# Error: No implementations provided for the following modules:
#          Unix referenced from /home/paus/.local/share/opam/toolstack-local-no-async/lib/inotify/inotify.cmxa(Inotify)

And this seems to be an easy enough fix, but I don't know whether I'm using it wrong

I'll be glad to add some missing dependency on my side. unix is already present in the dune dependency list of the binary, and adding base-unix to the opam dependency list doesn't fix it. If anybody is interested in looking into this, the branch that causes this is in xapi-project/xen-api@master...psafont:xen-api:no-async

[c-cube]

Thank you! :)