cpu/hotplug: Fix unused function warning

The cpuhp_is_ap_state() function is no longer called outside of the
CONFIG_SMP #ifdef section, causing a harmless warning:

kernel/cpu.c:129:13: error: 'cpuhp_is_ap_state' defined but not used [-Werror=unused-function]

This moves the function into the #ifdef to get a clean build again.

Fixes: 17a2f1ce

 ("cpu/hotplug: Merge cpuhp_bp_states and cpuhp_ap_states")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: https://lkml.kernel.org/r/20180315153829.3819606-1-arnd@arndb.de