Add dependency to odoc

Author: raphael-proust

lwt.opam

@@ -24,6 +24,7 @@ depends: [
   "dune-configurator"
   "ocaml" {>= "4.08"}
   "ocplib-endian"
+  "odoc" {with-doc & >= "2.3.0" }
 
   # Until https://github.com/aantron/bisect_ppx/pull/327.
   # "bisect_ppx" {dev & >= "2.0.0"}