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

arch/alpha/kernel/process.c

@@ -9,6 +9,7 @@
  * This file handles the architecture-dependent parts of process handling.
  */
 
+#include <linux/cpu.h>
 #include <linux/errno.h>
 #include <linux/module.h>
 #include <linux/sched.h>