Suppress gcc warning on privates-only class

Since gcc 9 it warns about is_streamable to have only private methods.

include/fmt/ostream.h

@@ -70,6 +70,10 @@ void_t<> operator<<(std::basic_ostream<char, Traits>&, signed char);
 template <typename Traits>
 void_t<> operator<<(std::basic_ostream<char, Traits>&, unsigned char);
 
+#ifdef __GNUC__
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wctor-dtor-privacy"
+#endif
 // Checks if T has a user-defined operator<< (e.g. not a member of
 // std::ostream).
 template <typename T, typename Char> class is_streamable {
@@ -87,6 +91,9 @@ template <typename T, typename Char> class is_streamable {
  public:
   static const bool value = result::value;
 };
+#ifdef __GNUC__
+#  pragma GCC diagnostic pop
+#endif
 
 // Write the content of buf to os.
 template <typename Char>