chore: gpu_bitunpack bench is behind cfg flag (#5369)

Signed-off-by: Robert Kruszewski <github@robertk.io>

Author: robert3005

vortex-gpu/benches/gpu_bitunpack.rs

@@ -2,6 +2,7 @@
 // SPDX-FileCopyrightText: Copyright the Vortex contributors
 
 #![allow(clippy::unwrap_used)]
+#![cfg(gpu_unstable)]
 
 use std::sync::Arc;
 use std::time::Duration;