re-enable lz4 compression for rocksdb on macOS and Linux

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

build/fbcode_builder/manifests/rocksdb

@@ -15,7 +15,7 @@ subdir = rocksdb-6.8.1
 
 [cmake.defines]
 WITH_SNAPPY=ON
-WITH_LZ4=OFF
+WITH_LZ4=ON
 WITH_TESTS=OFF
 WITH_BENCHMARK_TOOLS=OFF
 # We get relocation errors with the static gflags lib,