PM / Freezer: Fixup compile error of try_to_freeze_nowarn()

If FREEZER is not defined, the error as following will be throw
when compiled.
arch/arm/kernel/signal.c:645: error: implicit declaration of function
'try_to_freeze_nowarn'

Signed-off-by: Haifeng Li <omycle@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>