Summary:
Fixes compilation warnings (which are turned into compilation errors by default) when compiling with g++ option `-Wsuggest-override`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4272

Differential Revision: D9322556

Pulled By: siying

fbshipit-source-id: abd57a29ec8f544bee77c0bb438f31be830b7244