Ensure a CompletedIntentUpdate subclass is the final state

Change-Id: Icecf8a45fad7cb80f3953dcfc081c344d3fa0adb
diff --git a/core/net/src/main/java/org/onosproject/net/intent/impl/Withdrawn.java b/core/net/src/main/java/org/onosproject/net/intent/impl/Withdrawn.java
index 6ea20ae..b4864bc 100644
--- a/core/net/src/main/java/org/onosproject/net/intent/impl/Withdrawn.java
+++ b/core/net/src/main/java/org/onosproject/net/intent/impl/Withdrawn.java
@@ -20,7 +20,7 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 import static org.onosproject.net.intent.IntentState.WITHDRAWING;
 
-class Withdrawn implements CompletedIntentUpdate {
+class Withdrawn extends CompletedIntentUpdate {
 
     private final IntentData intentData;