plugins: starfield: enable USB audio

forgot about this...

Change-Id: Ib9d1451785390f1fabcaab679271510e76f9e330

apps/plugins/starfield.c

@@ -235,6 +235,12 @@ static int plugin_main(void)
 
             /* Get the peaks. ( Borrowed from vu_meter ) */
             static struct pcm_peaks peaks;
+#ifdef USB_ENABLE_AUDIO
+        if (rb->usb_audio_get_playing())
+            rb->mixer_channel_calculate_peaks(PCM_MIXER_CHAN_USBAUDIO,
+                                              &peaks);
+        else
+#endif
             rb->mixer_channel_calculate_peaks(PCM_MIXER_CHAN_PLAYBACK,
                                               &peaks);
             #define left_peak peaks.left

manual/plugins/starfield.tex

@@ -2,6 +2,7 @@
 \subsection{Starfield}
 \screenshot{plugins/images/ss-starfield}{Starfield}{img:starfield}
 Starfield simulation (like the classic screensaver).
+Speed reacts to playing audio, or can be manually adjusted.
 
 \begin{btnmap}
     \PluginRight{} / \PluginLeft