monopam: declare missing build deps (retry, mdx)
[retry] became a runtime dep when [ls_remote_exists] adopted
[Retry.with_retry_result]; the dune-project entry was missing so
[monopam lint] flagged it. The README mdx test stanza in [monopam/dune]
also requires [(using mdx 0.4)] in [dune-project] and [mdx :with-test]
in the opam depends list.