timber: add missing message to ListItemDueDate

Fixes display of fine with returned items.

timber/src/frontend/components/ListItemDueDate.js

@@ -55,6 +55,11 @@ export const messages = defineMessages({
     id: "UserLoans.dueDatePassed",
     description: "The due date of a reservation was passed",
     defaultMessage: "Due date passed"
+  },
+  returned: {
+    id: "UserLoans.returned",
+    description: "When an item is returned",
+    defaultMessage: "Returned"
   }
 })