Expose SEEK_DATA and SEEK_HOLE on Android

[chenxiaolong]

What does this PR do

This exposes Whence::SeekData and Whence::SeekHole on Android. These have been supported in Android itself and in the libc crate for several years.

This is the libc commit that added SEEK_DATA and SEEK_HOLE to their tests on Android: https://github.com/rust-lang/libc/commit/aa1c8ea6ba7e2b5e01f70ee6468dacd7d4588844

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
  • I don't think any changes are needed
• [x] A change log has been added if this PR modifies nix's API

[chenxiaolong]

Hmm, it doesn't look like the failures from the CI runs for the merge queue are related to my changes.