gnrc_netif: guard usage of gnrc_ipv6 member
gnrc_netif: guard usage of gnrc_ipv6 member

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