[posix] enable sntp client by default (#3289)
[posix] enable sntp client by default (#3289)


turon/openthread,abtink/openthread,librasungirl/openthread,erja-gp/openthread,jwhui/openthread,lanyuwen/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,turon/openthread,bukepo/openthread,lanyuwen/openthread,erja-gp/openthread,turon/openthread,bukepo/openthread,lanyuwen/openthread,lanyuwen/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,bukepo/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,erja-gp/openthread,lanyuwen/openthread,abtink/openthread,chshu/openthread,chshu/openthread,turon/openthread,librasungirl/openthread,librasungirl/openthread,openthread/openthread,srickardti/openthread,librasungirl/openthread,chshu/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,erja-gp/openthread,erja-gp/openthread,erja-gp/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread