Fix default value compile issues

src/core/lib/surface/completion_queue.h

@@ -77,7 +77,7 @@ bool grpc_cq_begin_op(grpc_completion_queue* cc, void* tag);
    grpc_cq_begin_op */
 void grpc_cq_end_op(grpc_completion_queue* cc, void* tag, grpc_error* error,
                     void (*done)(void* done_arg, grpc_cq_completion* storage),
-                    void* done_arg, grpc_cq_completion* storage, bool internal);
+                    void* done_arg, grpc_cq_completion* storage, bool internal = false);
 
 grpc_pollset* grpc_cq_pollset(grpc_completion_queue* cc);