timers: Fixup allmodconfig build issue

Adds missed EXPORT_SYMBOL lines that cause the following build
failures with allmodconfig:
ERROR: "timerqueue_add" [drivers/rtc/rtc-core.ko] undefined!
ERROR: "timerqueue_getnext" [drivers/rtc/rtc-core.ko] undefined!
ERROR: "timerqueue_del" [drivers/rtc/rtc-core.ko] undefined!

Reported-by: Ingo Molnar <mingo@elte.hu>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>