ocaml-rpmsg: defer README example via let run ()
The block called [Eio_main.run] then [Rpmsg.Ctrl.open_] which opened
[/dev/rpmsg_ctrl0] under mdx execution, failing on every machine
without a remoteproc. Wrap as [run () = ...] and add the [eio*] /
[fmt] mdx libraries that the example references.