[kernel] Empty wait_queue after calling clear()

Bugfix - wait_queue::clear() was not emptying out its util::deque after
waking all the threads, so it would just grow forever.

src/kernel/wait_queue.cpp

@@ -58,4 +58,5 @@ wait_queue::clear(uint64_t value)
         if (!t->exited()) t->wake(value);
         t->handle_release();
     }
+    m_threads.clear();
 }