Add missing DNS_ZONEFLG_DUMPING to setmodtime()
It was found, that the original commit adding the setmodtime() was incompletely squashed and there was double check for DNS_ZONEFLG_NEEDDUMP instead of check for DNS_ZONEFLG_NEEDDUMP and DNS_ZONEFLG_DUMPING.
Change the duplicate check to DNS_ZONEFLG_DUMPING.