Allow akkounts to set private XML storage data

Enables kosmos/akkounts#116

site-cookbooks/kosmos-ejabberd/templates/ejabberd.yml.erb

@@ -195,6 +195,7 @@ api_permissions:
       - "add_rosteritem"
       - "delete_rosteritem"
       - "send_message"
+      - "private_set"
 
 language: "en"