Fix FileAssetReader clippy on Windows

[greeble-dev]

#22560 added this code:

.map(|file| GuardedFile {
    file,
    #[cfg(not(target_os = "windows"))]
    _guard,
    #[cfg(target_os = "windows")]
    _lifetime: PhantomData::default(),
})

But clippy says no!

error: use of `default` to create a unit struct                                                                                              
  --> crates\bevy_asset\src\io\file\file_asset.rs:94:28
   |
94 |                 _lifetime: PhantomData::default(),
   |                            ^^^^^^^^^^^-----------
   |                                       |
   |                                       help: remove this call to `default`
   |

Testing

Tested on Win10 only.

cargo run --example asset_loading