plugin api: Add audio_pre_ff_rewind to plugin's API

According to wps code audio_pre_ff_rewind function should be called
before any rewinding. It stops playback and automatically resumes it
after audio_ff_rewind call

So, let's add audio_pre_ff_rewind to plugin's API

Lua scipts were tested:
```lua
-- has issue with rewinding
rb.audio("ff_rewind", 0)
```

```lua
-- no issue with rewinding
rb.audio("pre_ff_rewind")
rb.audio("ff_rewind", 0)
```

Change-Id: I2ad6b9c396760b2086bc0a28633a1c80c3512739

firmware/export/audio.h

@@ -61,6 +61,7 @@ size_t audio_buffer_size(void);
 /* size of the buffer available for allocating memory from the audio buffer using core_*()
  * returns core_available() if audio buffer is not allocated yet */
 size_t audio_buffer_available(void);
+void audio_pre_ff_rewind(void);
 void audio_ff_rewind(long newpos);
 void audio_flush_and_reload_tracks(void);
 struct mp3entry* audio_current_track(void);