Add OCamlBuild dependency

OCamlBuild is know (since OCaml 4.03) delivered aside the compiler. The package
just installs it, when necessary.

opam

@@ -14,6 +14,7 @@ build: [
 install: [make "install"]
 remove: ["ocamlfind" "remove" "ocl"]
 depends: [
+  "ocamlbuild"
   "atdgen"
   "base-threads"
   "core" {>= "112.35.00"}