BUILD: fix 'make test' compile error

Without this, I get this:
```
In file included from test/runner.cpp:1340:
./test/audio/raw.h: In member function ‘void RawStreamTestSuite::readBufferTestTemplate(int, int, bool, bool)’:
./test/audio/raw.h:17: error: invalid use of incomplete type ‘struct Audio::SeekableAudioStream’
./audio/decoders/raw.h:38: error: forward declaration of ‘struct Audio::SeekableAudioStream’
./test/audio/raw.h:19: error: invalid use of incomplete type ‘struct Audio::SeekableAudioStream’
./audio/decoders/raw.h:38: error: forward declaration of ‘struct Audio::SeekableAudioStream’
...
```