fix compile for VS2015
fix compile for VS2015

Summary:
Without the cast, the build will break on Windows.
Closes https://github.com/facebook/rocksdb/pull/1982

Differential Revision: D4690462

Pulled By: ajkr

fbshipit-source-id: c493b6c

SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb