MAGETWO-92408: [Bamboo] Strict Type Static Failure

- Add strict type declaration to top of file

Author: tjwiebell

app/code/Magento/PageBuilder/Block/Adminhtml/ContentType/Edit/CloseButton.php

@@ -3,6 +3,8 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
+
 namespace Magento\PageBuilder\Block\Adminhtml\ContentType\Edit;
 
 use Magento\Framework\View\Element\UiComponent\Control\ButtonProviderInterface;