Add ocaml-rrdd-plugin.spec.in

This is a standalone library for creating rrdd plugins, extracted from
the xenserver/rrdd-plugins repo.
This package also needs to Require:

forkexec-devel%{?_isa}
ocaml-stdext-devel%{?_isa}
ocaml-xcp-rrd-devel%{?_isa}
ocaml-rrd-transport-devel%{?_isa}

Basically, the package has to "BuildRequire" everything that's listed as a requirement in the _oasis file, otherwise it can't build.  The "-devel" subpackage has to "Require" everything that's listed as a requirement in the META file.   This is so that when you yum install foo-devel you pull in all the other -devel packages which are needed to build against the library - without it findlib will complain.