Replace virtual dispatch with normal functions in buffers

include/fmt/os.h

@@ -377,9 +377,10 @@ struct ostream_params {
 };
 
 class file_buffer final : public buffer<char> {
+ private:
   file file_;
 
-  FMT_API void grow(size_t) override;
+  FMT_API static void grow(buffer<char>& buf, size_t);
 
  public:
   FMT_API file_buffer(cstring_view path, const ostream_params& params);