gnrc_ipv6_nib: don't pull in RDNSS support with sock_dns
gnrc_ipv6_nib: don't pull in RDNSS support with sock_dns

Just because a user pulls in the DNS client (`sock_dns`) doesn't mean
they want to pull in RDNSS option support as well. Besides, since
`gnrc_ipv6_nib_dns` depends on `gnrc_ipv6_nib`, this forms a circular
dependency that really isn't necessary.

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