Core/SmartScripts: Add SMART_TARGET_LOOT_RECIPIENTS. Does what it says on the tin.

Use this new target type to fix The Rider of Blood, The Rider of Frost, The Rider of the Unholy. Closes #17817.

src/server/game/AI/SmartScripts/SmartScriptMgr.cpp

@@ -374,6 +374,7 @@ bool SmartAIMgr::IsTargetValid(SmartScriptHolder const& e)
         case SMART_TARGET_CLOSEST_ENEMY:
         case SMART_TARGET_CLOSEST_FRIENDLY:
         case SMART_TARGET_STORED:
+        case SMART_TARGET_LOOT_RECIPIENTS:
             break;
         default:
             TC_LOG_ERROR("sql.sql", "SmartAIMgr: Not handled target_type(%u), Entry %d SourceType %u Event %u Action %u, skipped.", e.GetTargetType(), e.entryOrGuid, e.GetScriptType(), e.event_id, e.GetActionType());