docs: add missing file header

Signed-off-by: Thierry Bugier <tbugier@teclib.com>

ajax/commontree.php

@@ -1,4 +1,33 @@
 <?php
+/**
+ * ---------------------------------------------------------------------
+ * Formcreator is a plugin which allows creation of custom forms of
+ * easy access.
+ * ---------------------------------------------------------------------
+ * LICENSE
+ *
+ * This file is part of Formcreator.
+ *
+ * Formcreator is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * Formcreator is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Formcreator. If not, see <http://www.gnu.org/licenses/>.
+ * ---------------------------------------------------------------------
+ * @copyright Copyright © 2011 - 2019 Teclib'
+ * @license   http://www.gnu.org/licenses/gpl.txt GPLv3+
+ * @link      https://github.com/pluginsGLPI/formcreator/
+ * @link      https://pluginsglpi.github.io/formcreator/
+ * @link      http://plugins.glpi-project.org/#/plugin/formcreator
+ * ---------------------------------------------------------------------
+ */
 include ('../../../inc/includes.php');
 
 // Check required parameters