Fix unused_attributes warning

```
warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![foo]`
 --> crates/libcgroups/src/v2/devices/mod.rs:8:22
  |
8 | #[cfg_attr(coverage, feature(no_coverage))]
  |                      ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_attributes)]` on by default
```

Signed-off-by: Taiki Endo <te316e89@gmail.com>

crates/libcgroups/src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(coverage, feature(no_coverage))]
+
 //! Control groups provide a way of controlling groups of processes.
 //! Examples: controlling resource limits, execution priority, measuring resource usage,
 //! freezing, checkpointing and restarting groups of processes.

crates/libcgroups/src/v2/devices/mod.rs

@@ -5,7 +5,6 @@ pub mod program;
 
 #[cfg(test)]
 #[allow(clippy::too_many_arguments)]
-#[cfg_attr(coverage, feature(no_coverage))]
 pub mod mocks;
 
 pub use controller::Devices;