write_floating_seconds: Fall back to ::round (#3343)

On some toolchains, `std::round` is not available.

Fixes #3342

include/fmt/chrono.h

@@ -1168,10 +1168,13 @@ void write_floating_seconds(memory_buffer& buf, Duration duration,
   auto val = duration.count();
 
   if (num_fractional_digits < 0) {
+    // For `std::round` with fallback to `round`:
+    // On some toolchains `std::round` is not available (e.g. GCC 6).
+    using namespace std;
     num_fractional_digits =
         count_fractional_digits<Duration::period::num,
                                 Duration::period::den>::value;
-    if (num_fractional_digits < 6 && static_cast<rep>(std::round(val)) != val)
+    if (num_fractional_digits < 6 && static_cast<rep>(round(val)) != val)
       num_fractional_digits = 6;
   }