GUACAMOLE-210: Implement AuthenticationProvider shutdown() function required due to GUACAMOLE-393.

extensions/guacamole-auth-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationProvider.java

@@ -107,4 +107,9 @@ public class OpenIDAuthenticationProvider implements AuthenticationProvider {
 
     }
 
+    @Override
+    public void shutdown() {
+        // Do nothing
+    }
+
 }