implements Serializable for that: noted an issue with tomcat

git-svn-id: https://svn.apache.org/repos/asf/archiva/redback/redback-core/trunk@1429446 13f79535-47bb-0310-9956-ffa450edef68

redback-authentication/redback-authentication-api/src/main/java/org/apache/archiva/redback/authentication/AuthenticationFailureCause.java

@@ -18,12 +18,16 @@ package org.apache.archiva.redback.authentication;
  * under the License.
  */
 
+import java.io.Serializable;
+
 /**
  * detail on possible authentication failure.
+ *
  * @author Olivier Lamy
  * @since 1.4-M4
  */
 public class AuthenticationFailureCause
+    implements Serializable
 {
     private int cause;