Fix variable does not need to be mutable warning for aio test

This fixes the following warning during run of cargo test

warning: variable does not need to be mutable
  --> test/sys/test_aio.rs:16:13
   |
16 | fn poll_aio(mut aiocb: &mut AioCb) -> Result<()> {
   |             ^^^^^^^^^
   |
   = note: #[warn(unused_mut)] on by default

Author: SanchayanMaity

test/sys/test_aio.rs

@@ -13,7 +13,7 @@ use std::{thread, time};
 use tempfile::tempfile;
 
 // Helper that polls an AioCb for completion or error
-fn poll_aio(mut aiocb: &mut AioCb) -> Result<()> {
+fn poll_aio(aiocb: &mut AioCb) -> Result<()> {
     loop {
         let err = aiocb.error();
         if err != Err(Error::from(Errno::EINPROGRESS)) { return err; };