sqlite: declare missing build deps (menhir, mdx)
[lib/dune] has a [(menhir ...)] stanza and [dune-project] declares
[(using menhir 3.0)], but [sqlite.opam] did not list [menhir] so
[dune build -p sqlite] failed with "Program menhir not found". Same
story for the README [(mdx)] stanza.
Also correct a stale comment: the SQL lexer uses [ocamllex] (see
[lib/lexer.mll]), not sedlex.