selabel: Fix clippy lint

[travier]

warning: manual case-insensitive ASCII comparison
   --> src/selabel.rs:248:16
    |
248 |             if key.trim().to_ascii_uppercase() == "SELINUXTYPE" {
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ignore_case_cmp
    = note: `#[warn(clippy::manual_ignore_case_cmp)]` on by default
help: consider using `.eq_ignore_ascii_case()` instead
    |
248 |             if key.trim().eq_ignore_ascii_case("SELINUXTYPE") {
    |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~