queue: added queue_shuffle_order_last()

This function shuffles the last song of a range.  This is used by
addSongToPlaylist().

src/queue.c

@@ -254,6 +254,13 @@ queue_shuffle_order(struct queue *queue)
 						  queue->length));
 }
 
+void
+queue_shuffle_order_last(struct queue *queue, unsigned start, unsigned end)
+{
+	queue_swap_order(queue, end - 1,
+			 g_rand_int_range(queue->rand, start, end));
+}
+
 void
 queue_shuffle_range(struct queue *queue, unsigned start, unsigned end)
 {