add 4.04 support in ppx/

Note that ppx_cstruct.404.ml is just an unchanged copy of
ppx_cstruct.403.ml. This seems to work well with the testsuite
provided. I used a plain file copy instead of a symlink as I don't know how
git symlinks are handled on Windows machines.

Having two files that are exactly the same means a fair amount of
redundancy, but I think this redundancy was chosen by the decision to
have a ppx_cstruct.$VERSION.ml file per OCaml version in the first
place. If one wants to reduce redundancy in the future, I guess that
a single file with preprocessor directives would make more sense.