sched/hrtick: Mark hrtick_clear() as always used

sched/hrtick: Mark hrtick_clear() as always used

This recent commit:

  96d1610e0b20b ("sched: Optimize hrtimer handling")

introduced a new build warning when !CONFIG_HOTPLUG_CPU
while SCHED_HRTIMERS=y [ == HIGH_RES_TIMERS=y ]:

  /tip.testing/kernel/sched/core.c:882:13: warning: ‘hrtick_clear’ defined but not used [-Wunused-function]

Mark this helper function as always-used, instead of complicating
the code with another obscure #ifdef.

Fixes: 96d1610e0b20b ("sched: Optimize hrtimer handling")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/177245077226.1647592.1821545206171336606.tip-bot2@tip-bot2