fix(tvix/nix-compat): Feature flag code only used in async

When only the wire feature was enabled two methods were unused and so
would faild `cargo check --no-default-features --features wire`.

This feature flags those two methods on async feature since that is the
only place they are used.

Change-Id: I6ec18a670e3c6e3ecee8d1417c99f1a5084e0ae7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12346
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: Brian Olsen <me@griff.name>
Tested-by: BuildkiteCI

tvix/nix-compat/src/wire/bytes/mod.rs

@@ -1,9 +1,12 @@
+#[cfg(feature = "async")]
+use std::mem::MaybeUninit;
 use std::{
     io::{Error, ErrorKind},
-    mem::MaybeUninit,
     ops::RangeInclusive,
 };
-use tokio::io::{self, AsyncReadExt, AsyncWriteExt, ReadBuf};
+#[cfg(feature = "async")]
+use tokio::io::ReadBuf;
+use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
 
 pub(crate) mod reader;
 pub use reader::BytesReader;
@@ -79,6 +82,7 @@ where
     Ok(buf)
 }
 
+#[cfg(feature = "async")]
 pub(crate) async fn read_bytes_buf<'a, const N: usize, R>(
     reader: &mut R,
     buf: &'a mut [MaybeUninit<u8>; N],
@@ -132,6 +136,7 @@ where
 }
 
 /// SAFETY: The bytes have to actually be initialized.
+#[cfg(feature = "async")]
 unsafe fn assume_init_bytes(slice: &[MaybeUninit<u8>]) -> &[u8] {
     &*(slice as *const [MaybeUninit<u8>] as *const [u8])
 }