sd-event: Fix systemd crash when using timer units.

There was a copy-paste error introduced in commit c2ba3ad6604ef2e189d7e0a36d6911116e84d3ab
which causes the following error when using timer units:

Assertion '(x->type == SOURCE_MONOTONIC && y->type == SOURCE_MONOTONIC) || (x->type == SOURCE_REALTIME && y->type == SOURCE_REALTIME)'
failed at src/libsystemd/sd-event/sd-event.c:264, function latest_time_prioq_compare(). Aborting.