`DbTool`: Import class `Zend_Db_Adapter_Pdo_Abstract`

Since `DbTool::zendconn` is of type `Zend_Db_Adapter_Pdo_Abstract`, it has to be imported.

modules/setup/library/Setup/Utils/DbTool.php

@@ -6,6 +6,7 @@ namespace Icinga\Module\Setup\Utils;
 use PDO;
 use PDOException;
 use LogicException;
+use Zend_Db_Adapter_Pdo_Abstract;
 use Zend_Db_Adapter_Pdo_Mysql;
 use Zend_Db_Adapter_Pdo_Pgsql;
 use Icinga\Util\File;