message: Fix build on Linux
diff: @@ -91,7 +91,7 @@ void append(prometheus::char_buffer& buf, double val, Ts&&... xs) {
  if (isnan(val)) {
  append(buf, "NaN"_sv);
  } else if (isinf(val)) {
  - if (signbit(val))
  + if (std::signbit(val))
  append(buf, "+Inf"_sv);
  else
  append(buf, "-Inf"_sv);
  