decoder_control: store GCond object, not a player_control

Remove the decoder dependency on player_control.  All player_control
was needed for is to signal the player thread, and we can do that with
a simple GCond as well.

src/player_thread.c

@@ -1024,7 +1024,7 @@ player_task(gpointer arg)
 {
 	struct player_control *pc = arg;
 
-	struct decoder_control *dc = dc_new(pc);
+	struct decoder_control *dc = dc_new(pc->cond);
 	decoder_thread_start(dc);
 
 	player_buffer = music_buffer_new(pc->buffer_chunks);