chore: Fix compilation warnings on non-Linux

[kjarosh]

This fixes the following warning on macOS & Windows (and probably others that we do not build for here).

warning: unused import: `futures::StreamExt`
 --> desktop/src/gui/theme.rs:5:5
  |
5 | use futures::StreamExt;
  |     ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: `ruffle_desktop` (bin "ruffle_desktop" test) generated 1 warning (run `cargo fix --bin "ruffle_desktop" --tests` to apply 1 suggestion)

[Dinnerbone]

Thank you, this has been bugging me but I keep forgetting to do anything about it :D

[torokati44]

Same! 😄💯

[Dinnerbone]

(should we do optional clippy runs on mac/windows, rust-stable?)

[torokati44]

should we do optional clippy runs on mac/windows, rust-stable?

Or at least pipe out the warnings/notes as "annotations" to the workflow runs/jobs somehow...?