GUACAMOLE-393: Add AuthenticationProvider shutdown hook.

extensions/guacamole-auth-duo/src/main/java/org/apache/guacamole/auth/duo/DuoAuthenticationProvider.java

@@ -102,4 +102,9 @@ public class DuoAuthenticationProvider implements AuthenticationProvider {
         return context;
     }
 
+    @Override
+    public void shutdown() {
+        // Do nothing
+    }
+
 }