_oasis: Include Nbd_lwt_common in the nbd-lwt library

The only important change is in the _oasis file, to include Nbd_lwt_common in the nbd-lwt library.

Without this change, RPM's auto-requires script will find that many of the files in the ocaml-nbd package import the Nbd_lwt_common interface, but the auto-provides script will find that no file in that package provides it.   Since no other package provides it either, ocaml-nbd will have an unfulfilled dependency and will not be installable.