scripts/checkpatch: add `__unused` to the `$Attribute` list

Add `__unused` to the `$Attribute` family along with its
`__maybe_unused`, `__always_unused` & `__used` brothers, so
that:

```c
__unused int ret;
```

is recognized as variable declaration, and doesn't raise
`LINE_SPACING` warning in CI.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>