Make classes derived from buffer<T> final to silence the virtual destructor warning. (#1937)

Co-authored-by: Bart Siwek <bsiwek@cisco.com>

include/fmt/os.h

@@ -378,7 +378,7 @@ struct ostream_params {
 static constexpr detail::buffer_size buffer_size;
 
 // A fast output stream which is not thread-safe.
-class ostream : private detail::buffer<char> {
+class ostream final : private detail::buffer<char> {
  private:
   file file_;