Summary:
mv port/dirent.h to port/port_dirent.h to avoid compile err when use port dir as header dir output
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5152
Differential Revision: D14779409
Pulled By: siying
fbshipit-source-id: d4162c47c979c6e8cc6a9e601802864ab3768ecb