Update thread_priority_greater function signature to explicitly signal unused parameter

src/threads/thread.c

@@ -603,7 +603,7 @@ allocate_tid (void)
    than that of the second list element's thread. */
 bool 
 thread_priority_greater (const struct list_elem *a, const struct list_elem *b,
-                         void *aux)
+                         void *aux UNUSED)
 {
   struct thread *ta = list_entry (a, struct thread, elem);
   struct thread *tb = list_entry (b, struct thread, elem);