Header file should not be executable (#7182)

Summary:
As title.
Undo file mode change in https://github.com/facebook/rocksdb/issues/6759 .

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7182

Reviewed By: ajkr

Differential Revision: D22786166

Pulled By: riversand963

fbshipit-source-id: 696903069acda42f26bbbf1f2875f5a08b761b42