[gtkada] Example GtkDial falis to compile

[Emmanuel Briot]

> I tried to compile the tutorial example gtkdial using GPS but gtk_dial.ads failed symmantic checking with:
> "Gtk_Update_Type" is undefined
> "Update_Continuous" is undefined
> "Timeout_Handler_Id"  is undefined
> 
> This seems hardly surprising as I can find none of these in the GtkAda_Documaentation dated 2016-03-25.

These examples have since been removed from the distribution since then, since indeed they
did not work anymore
 (GtkAda's github no longer has them).

regards
Emmanuel