Add ocaml-rrdd-plugin.spec.in

This is a standalone library for creating rrdd plugins, extracted from
the xenserver/rrdd-plugins repo.
Good question, not that I'm aware of. You can link to a range though e.g. https://github.com/xapi-project/xen-api/blob/master/ocaml/xapi/xapi.ml#L18-25