`AbstractWidget`: Import class `Zend_View_Abstract`

Since static property `AbstractWidget::view` is of type `Zend_View_Abstract` instance. The
class needs to be imported.

library/Icinga/Web/Widget/AbstractWidget.php

@@ -6,6 +6,7 @@ namespace Icinga\Web\Widget;
 use Icinga\Exception\ProgrammingError;
 use Icinga\Application\Icinga;
 use Exception;
+use Zend_View_Abstract;
 
 /**
  * Web widgets MUST extend this class