net/ipv4/addr.h: add ipv4_addr_is_multicast
net/ipv4/addr.h: add ipv4_addr_is_multicast

RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT