const qualify format function for systen_clock

include/fmt/chrono.h

@@ -1782,7 +1782,7 @@ struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
 
   template <typename FormatContext>
   auto format(std::chrono::time_point<std::chrono::system_clock> val,
-              FormatContext& ctx) -> decltype(ctx.out()) {
+              FormatContext& ctx) const -> decltype(ctx.out()) {
     return formatter<std::tm, Char>::format(localtime(val), ctx);
   }