Add missing history event field

Fixes #865

Author: quietust

df.history_event.xml

@@ -1126,6 +1126,7 @@
         <int32_t name='artifact_id' ref-target='artifact_record'/>
         <int32_t name='creator_unit_id' ref-target='unit' comment='the unit who created the artifact'/>
         <int32_t name='creator_hfid' ref-target='historical_figure' comment='the figure who created the artifact'/>
+        <int32_t name='entity' ref-target='historical_entity'/>
         <int32_t name='site' ref-target='world_site'/>
         <bitfield name='flags2' base-type='uint32_t' type-name='history_event_artifact_created_flag'/>
         <compound name='circumstance'> not a compound