Warnings during cargo check in macos-latest target

[CPerezz]

While running the CI in #800 the following was spoted.

Checking spotifyd v0.3.0 (/Users/runner/work/spotifyd/spotifyd)
warning: field is never read: `volume_controller`
Warning:    --> src/config.rs:594:5
    |
594 |     pub(crate) volume_controller: VolumeController,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted
Warning: 1 warning emitted
```

I think it would be a `good first issue` for new rustaceans.

[oriolval]

Let's go, I will be busy for the next days solving this issue. Thanks mate!

[robinvd]

should i assign you @oriolval

[CPerezz]

Sorry. This is a leftover of my work on #800. I mistakenly understood that the warn was there before when indeed it was produced by my conditional compilation code added.

I fixed this by adding:

#[cfg_attr(not(feature = "alsa_backend"), allow(dead_code))]
    pub(crate) volume_controller: VolumeController,

So I think this can be closed.
Sorry for the noise @oriolval and @robinvd