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

Susurrus/nix,nix-rust/nix,MarkusJais/nix-rust,geofft/nix-rust,xd009642/nix,carllerche/nix-rust,geofft/nix-rust,asomers/nix,MarkusJais/nix-rust,MarkusJais/nix-rust,asomers/nix,carllerche/nix-rust,nix-rust/nix,carllerche/nix-rust,xd009642/nix,Susurrus/nix,xd009642/nix,geofft/nix-rust,Susurrus/nix