Use proper macro to access TransactionId

In commit f10a025 I mistakenly used list_member_oid in a place
where list_member_xid is called for.  (Currently innocuous as both
typedefs are pretty much identical, but if we change either, it'll
become broken.)  Repair.

Author: Hou Zhijie <houzj.fnst@fujitsu.com>
Discussion: https://postgr.es/m/OS0PR01MB5716E2399494D4CB1A28A091942A9@OS0PR01MB5716.jpnprd01.prod.outlook.com

Author: alvherre

src/backend/nodes/list.c

@@ -747,7 +747,7 @@ list_member_xid(const List *list, TransactionId datum)
 
 	foreach(cell, list)
 	{
-		if (lfirst_oid(cell) == datum)
+		if (lfirst_xid(cell) == datum)
 			return true;
 	}