fix(linux): fix warnings from unexpected_cfgs lint on nightly

[rhysd]

Recently nightly rustc enabled unexpected_cfgs lint by default. Since this project uses non-standard cfg names, they started to cause warnings like below on Linux:

warning: unexpected `cfg` condition name: `gtk`
    --> src/lib.rs:1029:17
     |
1029 |       #[cfg(not(gtk))]
     |                 ^^^
     |
     = help: consider using a Cargo feature instead
     = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
              [lints.rust]
              unexpected_cfgs = { level = "warn", check-cfg = ['cfg(gtk)'] }
     = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(gtk)");` to the top of the `build.rs`
     = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

I didn't add a change file since this happens only on nightly yet. I confirmed those warnings were fixed on cargo +nightly check and also confirmed cargo check passed same as before.

Ref: rust-lang/rust#82450