Dummy META file installed since Jbuilder upgrade

[dra27]

Since the upgrade to Jbuilder in v1.6.0, cppo in opam installs an META file. Full details are in ocaml/dune#2353, the purpose of this issue is that the dune language version of cppo should be bumped when this is fixed in Dune.

(the context is Debian packaging of opam and ocaml/opam#3915)

[pmetzger]

Please do submit a pull request when appropriate.