time: Remove nonexistent function prototype

time: Remove nonexistent function prototype

The function clocksource_get_next() was removed in commit 75c5158f70
(timekeeping: Update clocksource with stop_machine), but the
prototype was not removed with it. Remove the prototype.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: <srv_heupstream@mediatek.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/1428674150-1780-1-git-send-email-yingjoe.chen@mediatek.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>