[cmake] [OCaml] Make ocamldoc targets depend on output files

OCaml doc builds fail without .cmi files, and .cmi files are collected
in ocaml_outputs. Therefore, make doc targets depend on ocaml_outputs as
well.

Fixes: https://llvm.org/bugs/show_bug.cgi?id=23777

Patch by Michał Górny <mgorny@gentoo.org>

llvm-svn: 239259