Summary:
It's impossible for a consumer of a released version of
rocksdb to do anything about this except not use it, and this particular
version of rocksdb ships with a number of shadow warnings.

Disable warning to error promotion.

See also: https://twitter.com/pcwalton/status/1201679307552083968

Reviewed By: chadaustin

Differential Revision: D18785637

fbshipit-source-id: 1db2b00b3c397d6c0b8f05b9d1c658877685c961