alpha/cpu: Expose arch_cpu_idle_dead()'s prototype declaration

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

Link: https://lore.kernel.org/r/b0405c2ac5686303b6026e1ac27cfd769b21a7d0.1676358308.git.jpoimboe@kernel.org


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