Fix build for current RocksDB HEAD

The prefix_extractor in the options was changed from a raw pointer
to a smart pointer. It happened in RocksDB commit
8d007b4aaf581a54756759c58270eb6a4d70f472 [1].

[1]: https://github.com/facebook/rocksdb/commit/8d007b4aaf581a54756759c58270eb6a4d70f472