MAINTAINERS: Add entry for kernel/time/alarmtimer.c

I've been missing patches against alarmtimer.c due to
a lack of a proper entry for it in the MAINTAINERS file.

So update MAINTAINERS to fix this, adding it in with the
timekeeping, ntp and core clocksource logic I share with
Thomas.

Signed-off-by: John Stultz <john.stultz@linaro.org>

MAINTAINERS

@@ -9334,7 +9334,7 @@ M:	Andreas Noever <andreas.noever@gmail.com>
 S:	Maintained
 F:	drivers/thunderbolt/
 
-TIMEKEEPING, CLOCKSOURCE CORE, NTP
+TIMEKEEPING, CLOCKSOURCE CORE, NTP, ALARMTIMER
 M:	John Stultz <john.stultz@linaro.org>
 M:	Thomas Gleixner <tglx@linutronix.de>
 L:	linux-kernel@vger.kernel.org
@@ -9347,6 +9347,7 @@ F:	include/uapi/linux/time.h
 F:	include/uapi/linux/timex.h
 F:	kernel/time/clocksource.c
 F:	kernel/time/time*.c
+F:	kernel/time/alarmtimer.c
 F:	kernel/time/ntp.c
 F:	tools/testing/selftests/timers/