GUACAMOLE-393: Add session invalidation hooks to UserContext.

extensions/guacamole-auth-ldap/src/main/java/org/apache/guacamole/auth/ldap/user/UserContext.java

@@ -229,4 +229,9 @@ public class UserContext implements org.apache.guacamole.net.auth.UserContext {
         return Collections.<Form>emptyList();
     }
 
+    @Override
+    public void invalidate() {
+        // Nothing to invalidate
+    }
+
 }