unix library missing in dune

[fblanqui]

dune clean; dune build generates the following warning with ocaml 5.1.1:

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).