make listener mutable again

[zestrada]

Not sure what's going on here, but I get this on some systems:

warning: variable does not need to be mutable
  --> src/main.rs:44:9
   |
44 |     let mut listener = VsockListener::bind(addr)?;
   |         ----^^^^^^^^
   |         |
   |         help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

but taking away mut as I did on the last commit leaves us with this on other systems

error[E0596]: cannot borrow `listener` as mutable, as it is not declared as mutable
  --> src/main.rs:59:29
   |
59 |         let (vsock, addr) = listener.accept().await?;
   |                             ^^^^^^^^^^^^^^^^^ cannot borrow as mutable
   |
help: consider changing this to be mutable
   |
44 |     let mut listener = VsockListener::bind(addr)?;
   |         +++

so this PR remakes listener mutable