rust: kunit: use crate-level mapping for `c_void`

Remove `use core::ffi::c_void`, which shadows `kernel::ffi::c_void`
brought in via `use crate::prelude::*`, to maintain consistency and
centralize the abstraction.

Since `kernel::ffi::c_void` is a straightforward re-export of
`core::ffi::c_void`, both are functionally equivalent. However, using
`kernel::ffi::c_void` improves consistency across the kernel's Rust code
and provides a unified reference point in case the definition ever needs
to change, even if such a change is unlikely.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089/topic/x/near/520452733


Signed-off-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://lore.kernel.org/r/20250528174953.2948570-1-y.j3ms.n@gmail.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>