[hotfix] Allow @internal annotation on fields

Author: zentol

flink-annotations/src/main/java/org/apache/flink/annotation/Internal.java

@@ -29,6 +29,6 @@
  * <p>Developer APIs are stable but internal to Flink and might change across releases.
  */
 @Documented
-@Target({ElementType.TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR})
+@Target({ElementType.TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.FIELD})
 @Public
 public @interface Internal {}