Fix not-backwards-compatible pg_upgrade test for prepared transactions.

There's no reason for this test to use the undocumented pg_prepared_xact()
function, when it can use the stable API pg_prepared_xacts instead.
Fixes breakage against 8.3, as reported by Justin Arnold.

contrib/pg_upgrade/check.c

@@ -532,7 +532,7 @@ check_for_prepared_transactions(ClusterInfo *cluster)
 
 	res = executeQueryOrDie(conn,
 							"SELECT * "
-							"FROM pg_catalog.pg_prepared_xact()");
+							"FROM pg_catalog.pg_prepared_xacts");
 
 	if (PQntuples(res) != 0)
 		pg_log(PG_FATAL, "The %s cluster contains prepared transactions\n",