opam: add some lower-bounds
opam: add some lower-bounds

We need a `cstruct` with a depopt on lwt. We need a newish `lwt`, so I
went for the Mirage minimum version of 2.4.3.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@unikernel.com>

mirage/ocaml-launchd,djs55/ocaml-launchd,djs55/ocaml-launchd,mirage/ocaml-launchd