x86/cpu: Expose arch_cpu_idle_dead()'s prototype definition

Include <linux/cpu.h> to make sure arch_cpu_idle_dead() matches its
prototype going forward.

Inspired-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230214083857.50163-1-philmd@linaro.org


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>