[posix] fix time sync issue (#4057)
[posix] fix time sync issue (#4057)

#4015 missed setting the timestamp when TIME_SYNC is enabled.
chshu/openthread,lanyuwen/openthread,abtink/openthread,turon/openthread,turon/openthread,srickardti/openthread,librasungirl/openthread,chshu/openthread,bukepo/openthread,jwhui/openthread,srickardti/openthread,srickardti/openthread,bukepo/openthread,bukepo/openthread,librasungirl/openthread,abtink/openthread,bukepo/openthread,turon/openthread,srickardti/openthread,librasungirl/openthread,jwhui/openthread,openthread/openthread,openthread/openthread,chshu/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,abtink/openthread,lanyuwen/openthread,jwhui/openthread,openthread/openthread,turon/openthread,lanyuwen/openthread,jwhui/openthread,librasungirl/openthread,lanyuwen/openthread,abtink/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread