Fix Node compile error on operator overload

src/core/lib/iomgr/executor/mpmcqueue.h

@@ -92,6 +92,7 @@ class MPMCQueue : public MPMCQueueInterface {
       next = nullptr;
       insert_time = gpr_now(GPR_CLOCK_PRECISE);
     }
+    GRPC_ABSTRACT_BASE_CLASS
   };
 
   struct Stats {             // Stats of queue

test/core/iomgr/mpmcqueue_test.cc

@@ -141,8 +141,6 @@ class WorkThread {
   void Start() { thd_.Start(); }
   void Join() { thd_.Join(); }
 
-  GRPC_ABSTRACT_BASE_CLASS
-
  private:
   void Run() {
     items_ = new WorkItem*[num_items_];