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

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

include/fmt/format.h

@@ -651,7 +651,7 @@ enum { inline_buffer_size = 500 };
  */
 template <typename T, size_t SIZE = inline_buffer_size,
           typename Allocator = std::allocator<T>>
-class basic_memory_buffer : public detail::buffer<T> {
+class basic_memory_buffer final : public detail::buffer<T> {
  private:
   T store_[SIZE];