m: Remove once_cell dependency

Removes the once_cell dependency, instead using std::sync::OnceLock and a
minimal polyfill for std::sync::LazyLock, which may be stabilized soon
(see rust-lang/rust#121377).

This should not require a bump in MSRV, as OnceLock was stabilized in 1.70,
which this crate is using.

src/platform_impl/linux/common/xkb/mod.rs

@@ -3,8 +3,8 @@ use std::os::raw::c_char;
 use std::ptr::{self, NonNull};
 use std::sync::atomic::{AtomicBool, Ordering};
 
+use crate::utils::Lazy;
 use log::warn;
-use once_cell::sync::Lazy;
 use smol_str::SmolStr;
 #[cfg(wayland_platform)]
 use std::os::unix::io::OwnedFd;