sys/net/fib: fix compilation with -Wcast-align
sys/net/fib: fix compilation with -Wcast-align

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