Add @FunctionalInterface to remaining public and protected interfaces

See gh-6857

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/TreeVisitor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -22,7 +22,6 @@ package org.springframework.boot.configurationprocessor.fieldvalues.javac;
  * @author Phillip Webb
  * @since 1.2.0
  */
-@FunctionalInterface
 interface TreeVisitor {
 
 	void visitVariable(VariableTree variable) throws Exception;