Fix uc_tailq_empty. It did not work prior to this.

include/ucore/tailq.h

@@ -59,7 +59,7 @@ static UC_INLINE void uc_tailq_init(struct TailQ *head)
  */
 static UC_INLINE int uc_tailq_empty(const struct TailQ *head)
 {
-    return head->next == head->prev;
+    return head->next == head;
 }
 
 /** Insert the entry between prev and next