mirage-fs-lwt: add a lower bound for lwt

otherwise it fails with:

```
  #=== ERROR while compiling mirage-fs-lwt.1.2.0 ================================#
  # context              2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.4.04.2 | pinned(https://github.com/mirage/mirage-fs/releases/download/v1.2.0/mirage-fs-v1.2.0.tbz)
  # path                 ~/.opam/4.04/.opam-switch/build/mirage-fs-lwt.1.2.0
  # command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p mirage-fs-lwt -j 71
  # exit-code            1
  # env-file             ~/.opam/log/mirage-fs-lwt-7-785134.env
  # output-file          ~/.opam/log/mirage-fs-lwt-7-785134.out
  ### output ###
  #       ocamlc lwt/.mirage_fs_lwt.objs/mirage_fs_lwt.{cmo,cmt} (exit 2)
  # (cd _build/default && /home/opam/.opam/4.04/bin/ocamlc.opt -w -40 -g -bin-annot -I lwt/.mirage_fs_lwt.objs -I /home/opam/.opam/4.04/lib/bytes -I /home/opam/.opam/4.04/lib/cstruct -I /home/opam/.opam/4.04/lib/fmt -I /home/opam/.opam/4.04/lib/lwt -I /home/opam/.opam/4.04/lib/mirage-device -I /home/opam/.opam/4.04/lib/mirage-fs -I /home/opam/.opam/4.04/lib/mirage-kv -I /home/opam/.opam/4.04/lib/mirage-kv-lwt -I /home/opam/.opam/4.04/lib/ocaml/camlp4 -I /home/opam/.opam/4.04/lib/ocplib-endian -I /home/opam/.opam/4.04/lib/optcomp -I /home/opam/.opam/4.04/lib/sexplib -no-alias-deps -o lwt/.mirage_fs_lwt.objs/mirage_fs_lwt.cmo -c -impl lwt/mirage_fs_lwt.ml)
  # File "lwt/mirage_fs_lwt.ml", line 17, characters 5-14:
  # Error: Unbound module Lwt.Infix
  #     ocamlopt lwt/.mirage_fs_lwt.objs/mirage_fs_lwt.{cmx,o} (exit 2)
  # (cd _build/default && /home/opam/.opam/4.04/bin/ocamlopt.opt -w -40 -g -I lwt/.mirage_fs_lwt.objs -I /home/opam/.opam/4.04/lib/bytes -I /home/opam/.opam/4.04/lib/cstruct -I /home/opam/.opam/4.04/lib/fmt -I /home/opam/.opam/4.04/lib/lwt -I /home/opam/.opam/4.04/lib/mirage-device -I /home/opam/.opam/4.04/lib/mirage-fs -I /home/opam/.opam/4.04/lib/mirage-kv -I /home/opam/.opam/4.04/lib/mirage-kv-lwt -I /home/opam/.opam/4.04/lib/ocaml/camlp4 -I /home/opam/.opam/4.04/lib/ocplib-endian -I /home/opam/.opam/4.04/lib/optcomp -I /home/opam/.opam/4.04/lib/sexplib -no-alias-deps -o lwt/.mirage_fs_lwt.objs/mirage_fs_lwt.cmx -c -impl lwt/mirage_fs_lwt.ml)
  # File "lwt/mirage_fs_lwt.ml", line 17, characters 5-14:
  # Error: Unbound module Lwt.Infix
```

Author: samoht

packages/mirage-fs-lwt/mirage-fs-lwt.1.2.0/opam

@@ -19,7 +19,7 @@ depends: [
   "mirage-fs" {>= "1.0.0" & < "3.0.0"}
   "mirage-kv" {>= "1.1.0"}
   "mirage-kv-lwt" {< "2.0.0"}
-  "lwt"
+  "lwt" {>= "2.4.7"}
   "cstruct" {>= "1.9.0"}
   "cstruct-lwt"
 ]