OCaml 5: explicit unix library loading

[hmedina]

OCaml 5 seems to have changed library loading, and the unix standard tool-chain needs an explicit invocation.

#=== ERROR while compiling kappa-binaries.4.1~dev =============================#
# context     2.1.2 | linux/x86_64 | ocaml.5.0.0 | pinned(git+https://github.com/Kappa-Dev/KappaTools.git#b4abe7ec)
# path        ~/.opam/default/.opam-switch/build/kappa-binaries.4.1~dev
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p kappa-binaries -j 11
# exit-code   1
# env-file    ~/.opam/log/kappa-binaries-2127-0a6acd.env
# output-file ~/.opam/log/kappa-binaries-2127-0a6acd.out
### output ###
# [...]
# (cd _build/default && /home/hmedina/.opam/default/bin/ocamlopt.opt -w -40 -open Kappa_grammar -open Kappa_terms -open Kappa_mixtures -open Kappa_parameters -open Kappa_generic_toolset -open Kappa_logging -open Kappa_cli -open Kappa_runtime -open Kappa_kasa_type_interface -g -I core/symmetries/.kappa_symmetries.objs/byte -I core/symmetries/.kappa_symmetries.objs/native -I /home/hmedina/.opam/d[...]
# 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).
# (cd _build/default && /home/hmedina/.opam/default/bin/ocamlopt.opt -w -40 -open Kappa_generic_toolset -open Kappa_mixtures -open Kappa_terms -open Kappa_runtime -open Kappa_grammar -open Kappa_cli -open Kappa_logging -open Kappa_parameters -open Kappa_staticanalyses -open Kappa_symmetries -open Kappa_reachability -open Kappa_kasa_export -open Kappa_kasa_type_interface -g -o core/odes/KaDE.exe[...]
# File "_none_", line 1:
# Error: No implementations provided for the following modules:
#          Unix referenced from core/cli/kappa_cli.cmxa(Kappa_cli__Kappa_files)