fix: correct arg type of prctl::set_timerslack

[SteveLauC]

What does this PR do

The ns argument of function nix::sys::prctl::set_timerslack() should have type c_ulong, before this PR, we are using an u64, which causes issue #2497.

This PR changes the type to c_ulong.

Closes #2497

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Well, that Cirrus CI issue happens again...

Using built-in Git...
Cloning refs/heads/gh-readonly-queue/master/pr-2505-7d75bbc9d6cfe91b903c91f39ed8b500ec65b8d1...
Failed to clone: couldn't find remote ref "refs/heads/gh-readonly-queue/master/pr-2505-7d75bbc9d6cfe91b903c91f39ed8b500ec65b8d1"!