Fix build with fmt11

Since fmt 11.0.0, formatter:format() is required to be const. Mark
all of the specializations as const to be compatible with fmt 11.

This change is implemented in the same spirit of 7725f6ed5a.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

include/util/format.hpp

@@ -92,7 +92,7 @@ struct formatter<pow_format> {
 template <>
 struct formatter<Glib::ustring> : formatter<std::string> {
   template <typename FormatContext>
-  auto format(const Glib::ustring& value, FormatContext& ctx) {
+  auto format(const Glib::ustring& value, FormatContext& ctx) const {
     return formatter<std::string>::format(static_cast<std::string>(value), ctx);
   }
 };