fix a header include
fix a header include

Summary:
need to consistently include "rocksdb/persistent_cache.h" to fix internal build
Closes https://github.com/facebook/rocksdb/pull/2034

Differential Revision: D4768101

Pulled By: ajkr

fbshipit-source-id: 2ecb07f

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