Fix GUI config crash due to missing Ed448.

How embarrassing - this morning's triumphant push of a shiny new
public-key method managed to break the entire GUI configuration system
so that it dereferences a null pointer during setup. That's what I get
for only testing the crypto side.

settings.c generates a preference list of host-key enum values that
included HK_ED448. So then hklist_handler() in config.c tries to look
that id up in its list of names, and doesn't find one, because I
forgot to add it there. Now reinstated.