d945483915 added a per-thread IOPL guard using std::sync::Once in a
#[cfg(target_os = "redox")] thread_local! (pci.rs:20) but did not import Once,
so redox-driver-sys fails to compile for x86_64-unknown-redox with E0425
"cannot find type Once" — breaking redox-drm (the /scheme/drm/card0 provider)
and driver-manager in the redbear-full build. Add the cfg-gated import.
Found while driving the redbear-full desktop build.
vasilito
1125c008bc