posix-timers: give lazy compilers some help optimizing code away

The OpenRISC compiler (so far) fails to optimize away a large portion of
code containing a reference to posix_timer_event in alarmtimer.c when
CONFIG_POSIX_TIMERS is unset.  Let's give it a direct clue to let the
build succeed.

This fixes
[linux-next:master 6682/7183] alarmtimer.c:undefined reference to `posix_timer_event'
reported by kbuild test robot.
Signed-off-by: NNicolas Pitre <nico@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/time/alarmtimer.c

@@ -516,7 +516,8 @@ static enum alarmtimer_restart alarm_handle_timer(struct alarm *alarm,
 
 	spin_lock_irqsave(&ptr->it_lock, flags);
 	if ((ptr->it_sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE) {
-		if (posix_timer_event(ptr, 0) != 0)
+		if (IS_ENABLED(CONFIG_POSIX_TIMERS) &&
+		    posix_timer_event(ptr, 0) != 0)
 			ptr->it_overrun++;
 	}