Virtual package for enabling the ctypes.foreign subpackage.

`ctypes-foreign` is just a virtual OPAM package that determines
whether the foreign subpackage should built as part of ctypes.
In order to actually get the ctypes package, you should also:

    opam install ctypes ctypes-foreign

You can verify the existence of the ocamlfind subpackage by:

    ocamlfind list | grep ctypes

Which should output something like:

    ctypes              (version: 0.4.1)
    ctypes.foreign      (version: 0.4.1)
    ctypes.foreign.base (version: 0.4.1)
    ctypes.foreign.threaded (version: 0.4.1)
    ctypes.foreign.unthreaded (version: 0.4.1)
    ctypes.stubs        (version: 0.4.1)
    ctypes.top          (version: 0.4.1)
