Fix build (#3136)

Summary:
Follow-up https://github.com/pytorch/audio/issues/3130

Pull Request resolved: https://github.com/pytorch/audio/pull/3136

Reviewed By: hwangjeff

Differential Revision: D43732991

Pulled By: mthrok

fbshipit-source-id: 2e8cb56d96e22546645c82eca362b3c4dcf9c78f

torchaudio/csrc/ffmpeg/CMakeLists.txt

@@ -19,6 +19,7 @@ set(
   stream_writer/encoder.cpp
   stream_writer/converter.cpp
   stream_writer/output_stream.cpp
+  stream_writer/audio_converter.cpp
   stream_writer/audio_output_stream.cpp
   stream_writer/video_converter.cpp
   stream_writer/video_output_stream.cpp