Make sure the spreed TURN server secret stays a secret

Signed-off-by: Joas Schilling <coding@schilljs.com>

lib/private/AppConfig.php

 <?php
 /**
+ * @copyright Copyright (c) 2017, Joas Schilling <coding@schilljs.com>
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
  * @author Arthur Schiwon <blizzz@arthur-schiwon.de>
@@ -42,6 +43,9 @@ class AppConfig implements IAppConfig {
 
 	/** @var array[] */
 	protected $sensitiveValues = [
+		'spreed' => [
+			'turn_server_secret',
+		],
 		'user_ldap' => [
 			'ldap_agent_password',
 		],