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

arch/x86/kernel/process.c

@@ -5,6 +5,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/prctl.h>
 #include <linux/slab.h>
 #include <linux/sched.h>