Add ocaml as a dependency of either

The package `either` builds with dune, and the dune executable doesn't
assume that an ocaml compiler is installed when building a project.
Packages that build with dune and need an ocaml compiler must depend
on the `ocaml` package.

Signed-off-by: Stephen Sherratt <[email protected]>

Author: gridbugs

packages/either/either.1.0.0/opam

@@ -13,6 +13,7 @@ doc: "https://mirage.github.io/either"
 bug-reports: "https://github.com/mirage/either/issues"
 depends: [
   "dune" {>= "2.0"}
+  "ocaml"
 ]
 build: [
   ["dune" "subst"] {dev}