udev: fix SECLABEL{selinux} issue (#15064)

Add SECLABEL{selinux}="some value" cause udevadm crash
systemd-udevd[x]: Worker [x] terminated by signal 11 (SEGV)

It happens since 25de7aa7 (Yu Watanabe 2019-04-25 01:21:11 +0200)
when udev rules processing changed to token model. Yu forgot store
attr to SECLABEL token so fix it.

(cherry picked from commit 0335d110)
(cherry picked from commit d58988be)