Add reference to iterator_traits

include/fmt/format.h

@@ -117,6 +117,7 @@ namespace std {
 template <> struct iterator_traits<fmt::appender> {
   using iterator_category = output_iterator_tag;
   using value_type = char;
+  using reference = char&;
   using difference_type = ptrdiff_t;
 };
 }  // namespace std