systemd.bbclass: Add /usr/lib/systemd to searchpaths as well
Some packages like lirc places its unit files into $systemd_user_unitdir
and also uses them in SYSTEMD_SERVICE list in recipe. This fails in
do_package
ERROR: Didn't find service unit 'lircmd.service', specified in SYSTEMD_SERVICE:lirc.
here lircmd.service is installed in /usr/lib/systemd/system/lircmd.service