[ticket/17362] Declare property to avoid PHP 8.2 deprecation notice

PHPBB-17362

phpBB/phpbb/extension/manager.php

@@ -31,6 +31,7 @@ class manager
 	protected $php_ext;
 	protected $extensions;
 	protected $extension_table;
+	protected $filesystem;
 	protected $phpbb_root_path;
 	protected $cache_name;