Add ocaml-rrdd-plugin.spec.in

This is a standalone library for creating rrdd plugins, extracted from
the xenserver/rrdd-plugins repo.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #66 from johnelse/add-ocaml-rrdd-plugin

Add ocaml-rrdd-plugin.spec.in