Adapt gix-config benches to changes in criterion

`criterion` 0.6.0 deprecates `criterion::black_box()`, preferring
`std::hint::black_box()` (which `criterion::black_box()` now uses).

This commit makes the recommended switch to
`std::hint::black_box()`, fixing new `clippy` errors.

Author: EliahKagan

gix-config/benches/large_config_file.rs

@@ -1,5 +1,6 @@
-use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use criterion::{criterion_group, criterion_main, Criterion};
 use gix_config::{parse::Events, File};
+use std::hint::black_box;
 
 fn gix_config(c: &mut Criterion) {
     c.bench_function("GitConfig large config file", |b| {