Fix librocksdb-sys build on macOS by installing RocksDB

Problem: librocksdb-sys v0.17.3+10.4.2 failed during cargo builds with a missing libclang.dylib error:

error: failed to run custom build command for `librocksdb-sys v0.17.3+10.4.2`

Caused by:
  process didn't exit successfully: `/path/to/project/target/debug/build/librocksdb-sys-*/build-script-build`
  --- stderr
  dyld: Library not loaded: @rpath/libclang.dylib

Fix:

brew install rocksdb

Re-run the build after installing.