x86: avoid section mismatch involving arch_register_cpu

Avoid section mismatch involving arch_register_cpu.

Marking arch_register_cpu as __init and removing the export
for non-hotplug-cpu configurations makes the following warning
go away:

Section mismatch in reference from the function
arch_register_cpu() to the function .devinit.text:register_cpu()
The function  arch_register_cpu() references
the function __devinit register_cpu().
This is often because arch_register_cpu lacks a __devinit
annotation or the annotation of register_cpu is wrong.

The only external user of arch_register_cpu in the tree is
in drivers/acpi/processor_core.c where it is guarded by
ACPI_HOTPLUG_CPU (which depends on HOTPLUG_CPU).

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
CC: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/asm-x86/cpu.h b/include/asm-x86/cpu.h
index 85ece5f..73f2ea8 100644
--- a/include/asm-x86/cpu.h
+++ b/include/asm-x86/cpu.h
@@ -10,8 +10,9 @@
 struct x86_cpu {
 	struct cpu cpu;
 };
-extern int arch_register_cpu(int num);
+
 #ifdef CONFIG_HOTPLUG_CPU
+extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
 #endif