sys/net/dns: mark dns_hdr_t as packed
sys/net/dns: mark dns_hdr_t as packed

This structure is used to parse data from unaligned buffers, so make
sure the compiler issues instructions suitable for unaligned memory
access.

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