Sprinkle around some library dependencies in dune files

[brk]

This enables a clean dune build, without any warnings of the form

File "_none_", line 1:
Alert ocaml_deprecated_auto_include:
OCaml's lib directory layout changed in 5.0. The unix subdirectory has been
automatically added to the search path, but you should add -I +unix to the
command-line to silence this alert (e.g. by adding unix to the list of
libraries in your dune file, or adding use_unix to your _tags file for
ocamlbuild, or using -package unix for ocamlfind).

[sipma]

Thank you!