sh/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.

Link: https://lore.kernel.org/r/3d9661e97828fb464a48d4becf18f12604831903.1676358308.git.jpoimboe@kernel.org


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