fix regression introduced in #760 (#764)

Author: mhowlett

src/Confluent.Kafka/IDeliveryHandler.cs

@@ -0,0 +1,24 @@
+// Copyright 2019 Confluent Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Refer to LICENSE for more information.
+
+
+namespace Confluent.Kafka
+{
+    internal interface IDeliveryHandler
+    {
+        void HandleDeliveryReport(DeliveryReport<Null, Null> deliveryReport);
+    }
+}

src/Confluent.Kafka/Producer.cs

@@ -40,11 +40,6 @@ internal class Config
             public Action<string> statisticsHandler;
         }
 
-        private interface IDeliveryHandler
-        {
-            void HandleDeliveryReport(DeliveryReport<Null, Null> deliveryReport);
-        }
-
         private ISerializer<TKey> keySerializer;
         private ISerializer<TValue> valueSerializer;
         private IAsyncSerializer<TKey> asyncKeySerializer;