Summary:
A few simple changes to allow RocksDB to be built on OpenBSD. Let me know if any further changes are needed.
Closes https://github.com/facebook/rocksdb/pull/3061

Differential Revision: D6138800

Pulled By: ajkr

fbshipit-source-id: a13a17b5dc051e6518bd56a8c5efd1d24dd81b0c