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

arch/sh/kernel/idle.c

@@ -4,6 +4,7 @@
  *
  *  Copyright (C) 2002 - 2009  Paul Mundt
  */
+#include <linux/cpu.h>
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/mm.h>