x86_32: select_idle_routine() must be __cpuinit

CONFIG_HOTPLUG_CPU=y:

WARNING: vmlinux.o(.text+0x1199a): Section mismatch: reference to .init.text.5:select_idle_routine (between 'init_intel' and 'init_nexgen')

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>