fix ctypes-foreign depexts to work with opam 2
fix ctypes-foreign depexts to work with opam 2

This commit syncs the ctypes-foreign.opam with the version
downstream in ocaml/opam-repository, and uses the opam 2 style
of specifying depexts.  This in turn makes depexts work with a
pinned ctypes-foreign, which is useful for automated CI of this
repository.

yallop/ocaml-ctypes,ocamllabs/ocaml-ctypes,yallop/ocaml-ctypes,ocamllabs/ocaml-ctypes