fix clang build
fix clang build

Summary:
override is missing for FilterV2
Closes https://github.com/facebook/rocksdb/pull/1606

Differential Revision: D4263832

Pulled By: IslamAbdelRahman

fbshipit-source-id: d8b337a

vmx/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb