sleep_until: disable allow(unused) on platform where its not unused

Co-authored-by: Jonas Böttiger <jonasboettiger@icloud.com>

Author: dvdsk

library/std/src/time.rs

@@ -408,8 +408,8 @@ impl Instant {
         self.0.checked_sub_duration(&duration).map(Instant)
     }
 
-    // used by platform specific `sleep_until` implementations.
-    #[allow(unused, reason = "not every platform has a specific `sleep_until`")]
+    // Used by platform specific `sleep_until` implementations such as the one used on Linux.
+    #[cfg_attr(not(target_os = "linux"), allow(unused, reason = "not every platform has a specific `sleep_until`"))]
     pub(crate) fn into_inner(self) -> time::Instant {
         self.0
     }