Fixed a compile warning in db_stress

Summary:
Fixed the following compile warning in db_stress:
error: 'OnCompactionCompleted' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

Test Plan: make db_stress

Reviewers: sdong, igor, anthony

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D39207

tools/db_stress.cc

@@ -807,7 +807,7 @@ class DbStressListener : public EventListener {
   }
 
   virtual void OnCompactionCompleted(
-      DB *db, const CompactionJobInfo& ci) {
+      DB *db, const CompactionJobInfo& ci) override {
     assert(db);
     assert(db->GetName() == db_name_);
     assert(IsValidColumnFamilyName(ci.cf_name));