fix(virtual_file): compile warnings on macos (#7525)

starting at commit
dbb0c967d5,
macOS reports warning for a few functions in the virtual file module.

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/virtual_file/owned_buffers_io/util/size_tracking_writer.rs

@@ -24,6 +24,7 @@ impl<W> Writer<W> {
 
     /// Returns the wrapped `VirtualFile` object as well as the number
     /// of bytes that were written to it through this object.
+    #[cfg_attr(target_os = "macos", allow(dead_code))]
     pub fn into_inner(self) -> (u64, W) {
         (self.bytes_amount, self.dst)
     }

pageserver/src/virtual_file/owned_buffers_io/write.rs

@@ -56,6 +56,7 @@ where
         self.buf()
     }
 
+    #[cfg_attr(target_os = "macos", allow(dead_code))]
     pub async fn flush_and_into_inner(mut self) -> std::io::Result<W> {
         self.flush().await?;
         let Self { buf, writer } = self;
@@ -70,6 +71,7 @@ where
             .expect("must not use after we returned an error")
     }
 
+    #[cfg_attr(target_os = "macos", allow(dead_code))]
     pub async fn write_buffered<S: IoBuf>(&mut self, chunk: Slice<S>) -> std::io::Result<(usize, S)>
     where
         S: IoBuf + Send,