rust: miscdevice: remove unnecessary import

`kernel::str::CStr` is included in the prelude.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250704-cstr-include-miscdevice-v1-1-bb9e9b17c892@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>