jvlc_audio: add missing function (get_track_count).

bindings/java/core/src/main/java/org/videolan/jvlc/Audio.java

@@ -72,6 +72,12 @@ public class Audio
         return jvlc.getLibvlc().libvlc_audio_get_track(mediaInstance.getInstance(), exception);
     }
 
+    public int getTrackCount(MediaPlayer mediaInstance)
+    {
+        libvlc_exception_t exception = new libvlc_exception_t();
+        return jvlc.getLibvlc().libvlc_audio_get_track_count(mediaInstance.getInstance(), exception);
+    }
+
     public void setTrack(MediaPlayer mediaInstance, int track)
     {
         libvlc_exception_t exception = new libvlc_exception_t();