Fix compilation with Lucene 6.2.

Original commit: elastic/x-pack-elasticsearch@a34002a11c

plugin/src/test/java/org/elasticsearch/xpack/security/authz/accesscontrol/SecurityIndexSearcherWrapperUnitTests.java

@@ -522,6 +522,11 @@ public class SecurityIndexSearcherWrapperUnitTests extends ESTestCase {
             assertTrue(seenLeaves.add(context.reader().getCoreCacheHelper().getKey()));
             return weight.bulkScorer(context);
         }
+
+        @Override
+        public boolean isCacheable(LeafReaderContext ctx) {
+            return true;
+        }
     }
 
     static class CreateScorerOnceQuery extends Query {