x86: fix do_fork_idle section mismatch

With CPU_HOTPLUG=n:

WARNING: vmlinux.o(.text+0x104f8): Section mismatch: reference to .init.text:fork_idle (between
'do_fork_idle' and 'lapic_timer_broadcast')

do_fork_idle() needs to be __cpuinit. It can be static as well.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/smpboot_64.c

@@ -526,7 +526,7 @@ struct create_idle {
 	int cpu;
 };
 
-void do_fork_idle(struct work_struct *work)
+static void __cpuinit do_fork_idle(struct work_struct *work)
 {
 	struct create_idle *c_idle =
 		container_of(work, struct create_idle, work);