MAGETWO-98251 Fix add to cart button on category page

Author: ptylek

app/design/frontend/Magento/luma/Magento_Catalog/web/css/source/module/_listings.less

@@ -360,7 +360,7 @@
                     position: absolute;
                     top: -2px;
                     width: 100%;
-                    z-index: 1;
+                    z-index: -1;
                 }
             }
         }