[gtkada] Illegal Ada code in src/gtk-marshallers.adb

[Arnaud Charlet]

> It is not important for me, but I thought I'd contribute somewhat
> after all the bug reports I submitted.  So, here goes.

Thanks. We'll likely have to make some changes (e.g. put these constants
in the private part), but that's a good starting point.

Although I find this Ada AI particularly painful, so we'll have to check
internally whether there is really no other way around it.

Arno