Add FunctionalInterface To X509PrincipalExtractor

Closes gh-16949

Signed-off-by: Max Batischev <mblancer@mail.ru>

web/src/main/java/org/springframework/security/web/authentication/preauth/x509/X509PrincipalExtractor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,7 @@ import java.security.cert.X509Certificate;
  *
  * @author Luke Taylor
  */
+@FunctionalInterface
 public interface X509PrincipalExtractor {
 
 	/**