Downgrade bindgen to 0.68

[jordalgo]

This is so we can package scx_utils into fedora without having to upgrade rust-bindgen
(https://bodhi.fedoraproject.org/updates/FEDORA-2023-18e7f124e1).

To make this happen we need to stop using the CargoCallbacks::new constructor which was added in 0.69. Old way seems legit according to the docs:
https://rust-lang.github.io/rust-bindgen/non-system-libraries.html

[jordalgo]

Now we get a warning if we build with 0.69:

[29/32] Generating rust/scx_utils/scx_utils with a custom command (wrapped by meson to set env)
warning: use of deprecated constant `bindgen::CargoCallbacks`: Use `CargoCallbacks::new()` instead. Please, check the documentation for further information.
  --> src/bpf_builder.rs:509:48
   |
509 |             .parse_callbacks(Box::new(bindgen::CargoCallbacks))
   |                                                ^^^^^^^^^^^^^^
   |
   = note: `#[warn(deprecated)]` on by default