When the target is inside the next long_period, the previous condition
was incorrect. The new condition should give the same result regardless
of whether there is a long_period tick coming soon or not.

Avoids a possible race condition in xtimer_set, xtimer_periodic where
the target time will have already passed when execution reaches the
_xtimer_now() call inside _xtimer_set_absolute.

The race condition is more easily triggered in xtimer_periodic because
of the longer execution time, but it is possible inside xtimer_set as
well if an interrupt arrives before the _xtimer_set_absolute call
and the ISR is non-trivial, and/or a context switch is triggered by
the ISR.