arm64: Fix -Wunused-function warning when !CONFIG_HOTPLUG_CPU

If CONFIG_HOTPLUG_CPU is n, gcc warns:

arch/arm64/kernel/smp.c:967:13: warning: ‘ipi_teardown’ defined but not used [-Wunused-function]
 static void ipi_teardown(int cpu)
             ^~~~~~~~~~~~

Use #ifdef guard this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200918123318.23764-1-yuehaibing@huawei.com