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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>