libudev: fix return of udev_monitor_filter_add_match_subsystem_devtype()

Follow-up for 71178426.

sd_device_monitor_filter_add_match_subsystem_devtype() now returns 1 to signify
that something was done, and 0 to signify that nothing was done, but
udev_monitor_filter_add_match_subsystem_devtype() needs to return 0 as documented.

udev_monitor_filter_add_match_tag() is adjusted to match.

This makes gdm start successfully here again.
Before, it would just not boot, with nothing very obvious in the logs:
gdm[1756]: Gdm: GdmDisplay: Session never registered, failing

Replaces #19171.