[gtkada] Two bugs with Gtk 1.2.8

[Arnaud Charlet]

> Body of Gtk.Label
> 
>    Needed "with Gtk.Widget;"
> 
> Body of Gtk.Option_Menu
>    Needed "with Gtk.Menu_Item;"

Thanks for reporting them, this is now fixed.
Apparently GNAT failed to detect these errors, are you using another
Ada compiler to build GtkAda by any chance ? ;-)

Arno