Add missing backslash for TokenList interfaces in stub (php#17009)

Discovered while trying to generate class synopses.

Author: ndossche

ext/dom/php_dom.stub.php

@@ -2096,7 +2096,7 @@ public function saveXmlFile(string $filename, int $options = 0): int|false {}
      * @not-serializable
      * @strict-properties
      */
-    final class TokenList implements IteratorAggregate, Countable
+    final class TokenList implements \IteratorAggregate, \Countable
     {
         /** @implementation-alias Dom\Node::__construct */
         private function __construct() {}