Summary:
FlushJobInfo and CompactionJobInfo are aggregates; we should use the
aggregate initialization syntax to ensure members (specifically those of
built-in types) are value-initialized.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5997

Test Plan: make check

Differential Revision: D18273398

Pulled By: ltamasi

fbshipit-source-id: 35b1a63ad9ca01605d288329858af72fffd7f392