Camlp4Top/Rprint.ml: support Osig_ellipsis

Osig_ellipsis was added in OCaml 4.03+dev by

    5adbb67a458f6f63deb1c1000f887ff87371c706
    Jacques Garrigue
    PR#6648: show_module should indicate its elision
    (add Osig_ellipsis to outcometree)
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15573y

It prevents building Camlp4 because of a non-exhaustive-matching
warning turned into an error.