Fix compile warning in compact_on_deletion_collector in some environment

Summary: Fix compile warning in compact_on_deletion_collector some environment

Test Plan: make

Reviewers: igor, sdong, anthony, IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43467

utilities/table_properties_collectors/compact_on_deletion_collector.h

@@ -70,12 +70,12 @@ class CompactOnDeletionCollector : public TablePropertiesCollector {
   }
 
   // The name of the properties collector can be used for debugging purpose.
-  virtual const char* Name() const {
+  virtual const char* Name() const override {
     return "CompactOnDeletionCollector";
   }
 
   // EXPERIMENTAL Return whether the output file should be further compacted
-  virtual bool NeedCompact() const {
+  virtual bool NeedCompact() const override {
     return need_compaction_;
   }