arm_arch_timer: include clocksource.h directly

This driver makes use of the clocksource code. Previously it had only
included the proper header indirectly, but that chain was inadvertently
broken by 74d23cc7

 "time: move the timecounter/cyclecounter code into its
own file."

This patch fixes the issue by including clocksource.h directly.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>