Fixed a compile warning in options_test.cc under clang
Fixed a compile warning in options_test.cc under clang

Summary:
Fixed the following compile warning in options_test.cc under clang
util/options_test.cc:94:12: error: 'Skip' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
    Status Skip(uint64_t n) {
           ^
./include/rocksdb/env.h:368:18: note: overridden virtual function is here
  virtual Status Skip(uint64_t n) = 0;
                 ^

Test Plan: options_test

Reviewers: igor, sdong, anthony, IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47763

jalexanderqed/rocksdb,facebook/rocksdb,hobinyoon/rocksdb,Vaisman/rocksdb,tsheasha/rocksdb,flabby/rocksdb,flabby/rocksdb,Vaisman/rocksdb,ryneli/rocksdb,Andymic/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,norton/rocksdb,SunguckLee/RocksDB,jalexanderqed/rocksdb,ryneli/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,facebook/rocksdb,Andymic/rocksdb,ryneli/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,vmx/rocksdb,flabby/rocksdb,jalexanderqed/rocksdb,flabby/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,hobinyoon/rocksdb,Andymic/rocksdb,bbiao/rocksdb,ryneli/rocksdb,tsheasha/rocksdb,Vaisman/rocksdb,flabby/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,norton/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,jalexanderqed/rocksdb,vmx/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,wenduo/rocksdb,norton/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,norton/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,SunguckLee/RocksDB,norton/rocksdb,SunguckLee/RocksDB,Vaisman/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,facebook/rocksdb,norton/rocksdb,ryneli/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,Andymic/rocksdb,jalexanderqed/rocksdb,hobinyoon/rocksdb,OverlordQ/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,facebook/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,flabby/rocksdb,wat-ze-hex/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,bbiao/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,ryneli/rocksdb,Vaisman/rocksdb,wenduo/rocksdb,wenduo/rocksdb,Vaisman/rocksdb,SunguckLee/RocksDB,hobinyoon/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,Vaisman/rocksdb,norton/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,vmx/rocksdb,tsheasha/rocksdb