Fix mix-up of 'FMT_BEGIN_EXPORT' and 'namespace detail'. (#3924)

include/fmt/format.h

@@ -4335,8 +4335,12 @@ FMT_CONSTEXPR FMT_INLINE auto native_formatter<T, Char, TYPE>::format(
                                          ctx);
   return write<Char>(ctx.out(), val, specs, ctx.locale());
 }
+
+FMT_END_EXPORT
 }  // namespace detail
 
+FMT_BEGIN_EXPORT
+
 template <typename Char>
 struct formatter<detail::float128, Char>
     : detail::native_formatter<detail::float128, Char,