add [open Types] back to cmmgen as it is used after the 4.00 merge

typing/types.ml was previously not used by cmmgen.ml in trunk, and the
[open] was thus removed by Alain in commit 12498. With the 4.00, some
of its constructors are now used. This does not prevent compilation
thanks to the new type-directed constructor resolution feature, but
still raises warning 40, which is best avoided.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13701 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02