gnrc/nib: don't advertise /128 prefix for auto-configuration
gnrc/nib: don't advertise /128 prefix for auto-configuration

A single IP address will be added as a /128 prefix.
It makes sense to advertise it, as neighbors can then know that
the address/prefix is on-link.

It does however not make sense to advertise the prefix as suitable
for auto-configuration.

Co-authored-by: Fabian Hüßler <13dea2555ad41961e875aa67990ab27d77d3a258@st.ovgu.de>

ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,miri64/RIOT