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>

include/linux/freezer.h

@@ -177,6 +177,7 @@ static inline int freeze_kernel_threads(void) { return -ENOSYS; }
 static inline void thaw_processes(void) {}
 static inline void thaw_kernel_threads(void) {}
 
+static inline bool try_to_freeze_nowarn(void) { return false; }
 static inline bool try_to_freeze(void) { return false; }
 
 static inline void freezer_do_not_count(void) {}