Add xml declaration for layout file

Author: Usik2203

app/code/Magento/CatalogWidget/view/frontend/layout/catalog_widget_product_list.xml

@@ -1,8 +1,4 @@
-<!--
-  ~ Copyright © Magento, Inc. All rights reserved.
-  ~ See COPYING.txt for license details.
-  -->
-
+<?xml version="1.0"?>
 <!--
   ~ Copyright © Magento, Inc. All rights reserved.
   ~ See COPYING.txt for license details.
@@ -14,4 +10,4 @@
             <block class="Magento\Framework\View\Element\Template" name="category.product.type.details.renderers.default" as="default"/>
         </block>
     </body>
-</page>
+</page>