GUACAMOLE-393: Add AuthenticationProvider shutdown hook.

extensions/guacamole-auth-header/src/main/java/org/apache/guacamole/auth/header/HTTPHeaderAuthenticationProvider.java

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