fix :bug: remove duplicate count in voucher offers (#4107)

src/oscar/apps/dashboard/vouchers/views.py

     def get_queryset(self):
         self.search_filters = []
         qs = self.model._default_manager.all()
-        qs = qs.annotate(num_offers=Count('offers'))
+        qs = qs.annotate(num_offers=Count('offers', distinct=True))
         qs = sort_queryset(qs, self.request,
                            ['num_basket_additions', 'num_orders', 'num_offers',
                             'date_created'],