Fix the require_once in install-helper.php so that it works when included from a plugin. Fixes #5090 for 2.3

git-svn-id: http://svn.automattic.com/wordpress/branches/2.3@6707 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/install-helper.php

@@ -1,5 +1,5 @@
 <?php
-require_once('../wp-config.php');
+require_once(dirname(dirname(__FILE__)).'/wp-config.php');
 $debug = 0;
 
 /**
@@ -149,4 +149,4 @@ if (check_column($wpdb->links, 'link_description', 'varchar(255)')) {
 }
 echo "</pre>";
 */
-?>
+?>