shell/gnrc_icmpv6_echo: fix build with USEMODULE += sock_dns
shell/gnrc_icmpv6_echo: fix build with USEMODULE += sock_dns

To be able to call sock_dns_query() in _configure(), we also
need to include the header file.

kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,basilfx/RIOT,RIOT-OS/RIOT,basilfx/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,smlng/RIOT,authmillenon/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,basilfx/RIOT,smlng/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,yogo1212/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,yogo1212/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,ant9000/RIOT,smlng/RIOT,yogo1212/RIOT,jasonatran/RIOT,smlng/RIOT,yogo1212/RIOT,kaspar030/RIOT,smlng/RIOT,kaspar030/RIOT,authmillenon/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT