sodium 0.2.1 is not compatible with ocaml 5

Fails to find bigarray:
```
=== ERROR while compiling sodium.0.2.1 =======================================#
 context              2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
 path                 ~/.opam/5.0/.opam-switch/build/sodium.0.2.1
 command              ~/.opam/opam-init/hooks/sandbox.sh build make
 exit-code            2
 env-file             ~/.opam/log/sodium-8-eef118.env
 output-file          ~/.opam/log/sodium-8-eef118.out
 mkdir -p $(dirname _build/lib/dllsodium.so)
 cc -shared -lsodium -o _build/lib/dllsodium.so
 ocamlbuild -use-ocamlfind lib/sodium.cma lib/sodium.cmxa
 ocamlfind ocamlopt unix.cmxa -I /home/opam/.opam/5.0/lib/ocamlbuild /home/opam/.opam/5.0/lib/ocamlbuild/ocamlbuildlib.cmxa -linkpkg myocamlbuild.ml /home/opam/.opam/5.0/lib/ocamlbuild/ocamlbuild.cmx -o myocamlbuild
 ocamlfind ocamldep -package bytes -package bigarray -package ctypes.foreign -modules lib/sodium.mli > lib/sodium.mli.depends
 + ocamlfind ocamldep -package bytes -package bigarray -package ctypes.foreign -modules lib/sodium.mli > lib/sodium.mli.depends
 ocamlfind: Package `bigarray' not found
 Command exited with code 2.
 + ocamlfind ocamlopt unix.cmxa -I /home/opam/.opam/5.0/lib/ocamlbuild /home/opam/.opam/5.0/lib/ocamlbuild/ocamlbuildlib.cmxa -linkpkg myocamlbuild.ml /home/opam/.opam/5.0/lib/ocamlbuild/ocamlbuild.cmx -o myocamlbuild
 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).
 make: *** [Makefile:2: all] Error 10
```

Signed-off-by: Marcello Seri <[email protected]>

Author: mseri

packages/sodium/sodium.0.2.1/opam

@@ -4,7 +4,7 @@ tags: ["org:mirage"]
 build: make
 remove: [["ocamlfind" "remove" "sodium"]]
 depends: [
-  "ocaml"
+  "ocaml" {< "5.0"}
   "base-bigarray"
   "base-bytes"
   "ocamlfind"