Add ocaml-rrdd-plugin.spec.in

This is a standalone library for creating rrdd plugins, extracted from
the xenserver/rrdd-plugins repo.
This can become the complement of the base package above:

%{_libdir}/ocaml/rrdd-plugin/*.a
%{_libdir}/ocaml/rrdd-plugin/*.cmx
%{_libdir}/ocaml/rrdd-plugin/*.cmxa
%{_libdir}/ocaml/rrdd-plugin/*.mli