make.dep/sntp: remove gnrc dependency
make.dep/sntp: remove gnrc dependency

This module should depend on sock_udp instead of gnrc_sock_udp.

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