Make enum generation private by default, export stub symbol

Make enum generation private by default, export stub symbol

When we added enum type generation, the generated symbols used
the `ostree_` prefix, and at the time that implied they were public.

So we started (if built with libsoup) exporting
`ostree_fetcher_config_flags_get_type`.

I think it's not worth confusing ABI checkers, so let's export the
dummy symbol forever, and switch enums to be private by default.

We should revisit this and also export some auto-generated enum
types for public enums, but that's a separate patch.

Closes: #296
Approved by: gatispaeglis