Summary:
I think LZ4 was disabled accidentally in our rocksdb getdeps build in
D21319896. Enable it again on macOS and Linux, because otherwise this
breaks people with EdenFS mounts containing LZ4-compressed proxy
hashes.

Reviewed By: xavierd

Differential Revision: D21990356

fbshipit-source-id: b9166c2992ae51f09de3fa9a4f114143aa008f43