Add ocaml-rrdd-plugin.spec.in

This is a standalone library for creating rrdd plugins, extracted from
the xenserver/rrdd-plugins repo.
You can use %{_libdir}/ocaml/rrdd-libs and then % exclude the files you don't want:
```
%exclude %{_libdir}/ocaml/rrdd-plugin/*.a
%exclude %{_libdir}/ocaml/rrdd-plugin/*.cmx
%exclude %{_libdir}/ocaml/rrdd-plugin/*.cmxa
%exclude %{_libdir}/ocaml/rrdd-plugin/*.mli
```