Add missing DNS_ZONEFLG_DUMPING to setmodtime()

Merged Ondřej Surý requested to merge ondrej/fix-missing-DNS_ZONEFLG_DUMPING-in-setmodtime into main

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.

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