lib: test_objpool: Avoid direct access to hrtimer clockbase

lib: test_objpool: Avoid direct access to hrtimer clockbase

The field timer->base->get_time is a private implementation detail and
should not be accessed outside of the hrtimer core.

Switch to the equivalent helper.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20250821-hrtimer-cleanup-get_time-v2-4-3ae822e5bfbd@linutronix.de