`unused_variables` warning when building with `filesystem_watcher` feature disabled

[sarkahn]

Objective

Fix unused_variables warning produced by bevy_asset when building as a local dependency with the filesystem_watcher feature disabled.

https://github.com/bevyengine/bevy/blob/main/crates/bevy_asset/src/io/file_asset_io.rs#L125-L143

For some reason this warning doesn't seem to be produced when using the crates.io dependency - I only observed it with a local copy of bevy 0.10

[alice-i-cecile]

@sarkahn can you try rebasing this? CI appears to be stuck.

[mockersf]

something like #[cfg_attr(not(feature = "filesystem_watcher"), allow(unused_variables))] would be more exact. I don't remember the exact syntax though