Fix test to be skipped if dom is not available

Author: cmb69

ext/libxml/tests/bug76777.phpt

@@ -3,6 +3,7 @@ Bug #76777 (first parameter of libxml_set_external_entity_loader callback undefi
 --SKIPIF--
 <?php
 if (!extension_loaded('libxml')) die('skip');
+if (!extension_loaded('dom')) die('skip dom extension not available');
 if (getenv("SKIP_ONLINE_TESTS")) die('skip online test');
 ?>
 --FILE--