Summary:
The Buck build of RocksDB supports LZ4, so if an Eden instance's
object cache ends up compressed with LZ4 compression, the opensource
getdeps build cannot open it. Enable LZ4 in the open source getdeps
build of rocksdb.

Reviewed By: pkaush

Differential Revision: D19961878

fbshipit-source-id: 9a4f37e12bd2b40a0c5dcf2dd1684876886fae26