MT#63171 AmEventQueue: virtual inherit refcnt

AmThread already inherits atomic_ref_cnt virtually. For cases of child
classes inheriting both AmThread and AmEventQueue, make sure only one
instance of atomic_ref_cnt is present.

Change-Id: I611db51f0b36e7cca2bcdfe8befa20bce975030b

core/AmEventQueue.h

@@ -61,7 +61,7 @@ class AmEventNotificationSink
  */
 class AmEventQueue
   : public AmEventQueueInterface,
-    public atomic_ref_cnt
+    virtual public atomic_ref_cnt
 {
 protected:
   AmEventHandler*           handler;