mark EnumUtil as @Private

Author: squito

core/src/main/java/org/apache/spark/util/EnumUtil.java

@@ -17,9 +17,9 @@
 package org.apache.spark.util;
 
 import com.google.common.base.Joiner;
-import org.apache.spark.annotation.DeveloperApi;
+import org.apache.spark.annotation.Private;
 
-@DeveloperApi
+@Private
 public class EnumUtil {
   public static <E extends Enum<E>> E parseIgnoreCase(Class<E> clz, String str) {
     E[] constants = clz.getEnumConstants();