HADOOP-18074 - fix compliation problem on branch-3.3

Author: lmccay

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/LdapGroupsMapping.java

@@ -460,7 +460,7 @@ private NamingEnumeration<SearchResult> lookupPosixGroup(SearchResult result,
    * @throws NamingException if unable to find group names
    */
   @VisibleForTesting
-  Set<String> lookupGroup(SearchResult result, DirContext c,
+  List<String> lookupGroup(SearchResult result, DirContext c,
       int goUpHierarchy)
       throws NamingException {
     List<String> groups = new ArrayList<>();