benches: box_blur: Replace deprecated function
benches: box_blur: Replace deprecated function

Criterion 0.3 deprecated Criterion::bench_function_over_inputs, to be
replaced by BenchmarkGroup. This commit replaces the deprecated call in
the suggested manner.

https://bheisler.github.io/criterion.rs/book/migrating_0_2_to_0_3.html

Closes https://gitlab.gnome.org/GNOME/librsvg/-/issues/603

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg