[ticket/10058] Initialize $phpbb_root_path in mysql_upgrader.php.

Since knowledge base instructions tell users to place this script
in the root of the forum, use './' as phpbb root path. Actual
initialization code copied from check_flash_bbcodes.php.

PHPBB3-10058

phpBB/develop/mysql_upgrader.php

@@ -21,6 +21,7 @@
 die("Please read the first lines of this script for instructions on how to enable it");
 
 define('IN_PHPBB', true);
+$phpbb_root_path = (defined('PHPBB_ROOT_PATH')) ? PHPBB_ROOT_PATH : './';
 $phpEx = substr(strrchr(__FILE__, '.'), 1);
 include($phpbb_root_path . 'common.' . $phpEx);