Build fails on 4.09

[aantron]

Seeing this error with wikidoc master:

#=== ERROR while compiling wikidoc.dev ========================================#
# context              2.0.6 | linux/x86_64 | ocaml-base-compiler.4.09.0 | pinned(git+https://github.com/ocsigen/wikidoc.git#8c10fd29)
# path                 ~/build/ocsigen/lwt/_opam/.opam-switch/build/wikidoc.dev
# command              /usr/bin/make BINDIR=/home/travis/build/ocsigen/lwt/_opam/bin
# exit-code            2
# env-file             ~/.opam/log/wikidoc-24055-b6f8d4.env
# output-file          ~/.opam/log/wikidoc-24055-b6f8d4.out
### output ###
# [...]
# ocamlfind ocamlc -c -I /home/travis/build/ocsigen/lwt/_opam/lib/ocaml/ocamldoc -I /home/travis/build/ocsigen/lwt/_opam/lib/ocaml/compiler-libs/ -I /home/travis/build/ocsigen/lwt/_opam/lib/ocaml/compiler-libs/parsing -I /home/travis/build/ocsigen/lwt/_opam/lib/ocaml/compiler-libs/typing odoc_import.ml
# findlib: [WARNING] Interface topdirs.cmi occurs in several directories: /home/travis/build/ocsigen/lwt/_opam/lib/ocaml, /home/travis/build/ocsigen/lwt/_opam/lib/ocaml/compiler-libs
# File "odoc_import.ml", line 175, characters 13-35:
# 175 |     | Types.(Mty_functor (Unit, mt)) -> Mty_functor (Unit, iter mt)
#                    ^^^^^^^^^^^^^^^^^^^^^^
# Error: The constructor Mty_functor expects 3 argument(s),
#        but is applied here to 2 argument(s)
# Makefile:45: recipe for target 'odoc_import.cmo' failed
# make[1]: *** [odoc_import.cmo] Error 2
# make[1]: Leaving directory '/home/travis/build/ocsigen/lwt/_opam/.opam-switch/build/wikidoc.dev/src/odoc_wiki'
# Makefile.odoc:11: recipe for target 'all' failed
# make: *** [all] Error 2