Update ext/xmlreader dependencies (#14572)

- ext/dom is optional (using HAVE_DOM for cases when dom is build as
  a shared extension to make it required in that case)
- ext/libxml is required

ext/xmlreader/php_xmlreader.c

@@ -277,6 +277,9 @@ static xmlRelaxNGPtr _xmlreader_get_relaxNG(char *source, size_t source_len, siz
 #endif
 
 static const zend_module_dep xmlreader_deps[] = {
+#ifdef HAVE_DOM
+	ZEND_MOD_REQUIRED("dom")
+#endif
 	ZEND_MOD_REQUIRED("libxml")
 	ZEND_MOD_END
 };