time/jiffies: Make clocksource_jiffies static

Commit f1b82746 ("clocksource: Cleanup clocksource selection") removed all
external references to clocksource_jiffies so there is no need to have the
symbol globally visible.

Fixes the following sparse warning:
  CHECK   kernel/time/jiffies.c kernel/time/jiffies.c:61:20: warning: symbol 'clocksource_jiffies' was not declared. Should it be static?

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>