xtimer/xtimer.c: sched_switch instead of yield

because of pr RIOT-OS#11759: not all boards check for is_in_irq when thread_yield_higher

sys/xtimer/xtimer.c

@@ -257,7 +257,7 @@ static void _mutex_timeout(void *arg)
         }
         sched_set_status(mt->thread, STATUS_PENDING);
         irq_restore(irqstate);
-        thread_yield_higher();
+        sched_switch(mt->thread->priority);
         return;
     }
     irq_restore(irqstate);