gpu: nova-core: use core library's CStr instead of kernel one

gpu: nova-core: use core library's CStr instead of kernel one

The kernel's own CStr type has been replaced by the one in the core
library, and is now an alias to the latter. Change our imports to
directly reference the actual type.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260217-nova-misc-v3-7-b4e2d45eafbc@nvidia.com
[acourbot@nvidia.com: remove unneeded imports reorganization in
firmware/gsp.rs]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>