[FIX] web: Dashboards error

Steps to reproduce:
Go to any pivot view (for example Sales > Report > Sales) and add
the current view to your personnal dashboard (in the search view: Favourites > Add to my Dashboard)
Go to the dashboard, traceback because $buttons was not defined.

opw:707296

addons/web/static/src/js/views/pivot_view.js

@@ -57,6 +57,7 @@ var PivotView = View.extend({
         this.headers = {};
         this.cells = {};
         this.has_data = false;
+        this.$buttons = $();
 
         this.last_header_selected = null;
         this.sorted_column = {};