Summary:
Fix `error: ‘FALLOC_FL_KEEP_SIZE’` undeclared error in `io_posix.cc` during Vagrant build in CentOS as per issue https://github.com/facebook/rocksdb/issues/5599
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5614

Differential Revision: D17217960

fbshipit-source-id: ef736c51b16833107fd9ccc7917ed1def2a8d02c