[ticket/17176] Add more test stubs for psalm

PHPBB3-17176

build/psalm/stubs/gd.php

@@ -0,0 +1,15 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+/** @link https://www.php.net/manual/en/image.constants.php */
+define('IMG_COLOR_STYLED', -2);