x86: apic_64.c - setup_APIC_timer has to be __cpuinit function

There is no need to hold this code if CPU_HOTPLUG is not
defined.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>