GUACAMOLE-197: Add override for module shutdown() method.

extensions/guacamole-auth-radius/src/main/java/org/apache/guacamole/auth/radius/RadiusAuthenticationProvider.java

@@ -68,6 +68,11 @@ public class RadiusAuthenticationProvider implements AuthenticationProvider {
         return null;
     }
 
+    @Override
+    public void shutdown() {
+        // Do nothing
+    }
+
     @Override
     public AuthenticatedUser authenticateUser(Credentials credentials) throws GuacamoleException {