DistributedFlowRuleStore: register FlowRuleEvent

Change-Id: I1f904a283c8e619a6ec9094d62ddfba44a41c119
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/flow/impl/DistributedFlowRuleStore.java b/core/store/dist/src/main/java/org/onlab/onos/store/flow/impl/DistributedFlowRuleStore.java
index 8126554..1961c8c 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/flow/impl/DistributedFlowRuleStore.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/flow/impl/DistributedFlowRuleStore.java
@@ -158,6 +158,7 @@
             serializerPool = KryoNamespace.newBuilder()
                     .register(DistributedStoreSerializers.STORE_COMMON)
                     .nextId(DistributedStoreSerializers.STORE_CUSTOM_BEGIN)
+                    .register(FlowRuleEvent.class)
                     .build();
         }
     };