systemd.bbclass: Add /usr/lib/systemd to searchpaths as well

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

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 12808a4159835b67d8d53d32bc9135811701a779)
Signed-off-by: Steve Sakoman <steve@sakoman.com>