Fix a compile warning in listener_test.cc

Summary:
Fixed the following compile warning in listener_test.cc:
db/listener_test.cc:214:8: error: 'OnTableFileCreated' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
14:16:46   void OnTableFileCreated(

Test Plan:
make listener_test

Reviewers: sdong, igor

Subscribers: leveldb

db/listener_test.cc

@@ -212,7 +212,7 @@ TEST_F(EventListenerTest, OnSingleDBCompactionTest) {
 class TestFlushListener : public EventListener {
  public:
   void OnTableFileCreated(
-      const TableFileCreationInfo& info) {
+      const TableFileCreationInfo& info) override {
     db_name_ = info.db_name;
     cf_name_ = info.cf_name;
     file_path_ = info.file_path;