[FIX] point_of_sale: set CSRF token

The introduction of the CSRF tokens in
8932c52e67 broke invoicing from the Point
of Sale. chrome.do_action('point_of_sale.pos_invoice_report') would
fail because no CSRF token is set in the POS. This fixes it using a
similar approach as in the website module.

addons/point_of_sale/views/templates.xml

@@ -46,8 +46,13 @@
                 });
                 return web_client;
             });
-        </script>
 
+            odoo.define('web.csrf', function (require) {
+                var token = "<t t-esc="request.csrf_token(None)"/>";
+                require('web.core').csrf_token = token;
+                require('qweb').default_dict.csrf_token = token;
+            });
+        </script>
     </head>
     <body>
         <div class='openerp openerp_webclient_container'>