Fix Windows build

format.cc

@@ -430,7 +430,7 @@ int fmt::internal::UTF16ToUTF8::convert(fmt::WStringRef s) {
 void fmt::WindowsError::init(
     int error_code, StringRef format_str, ArgList args) {
   error_code_ = error_code;
-  Writer w;
+  MemoryWriter w;
   internal::format_windows_error(w, error_code, format(format_str, args));
   std::runtime_error &base = *this;
   base = std::runtime_error(w.str());