package/lxc: add systemd optional dependency

Fix the following build failure raised since bump to version 5.0.1 in
commit db199980:

Run-time dependency systemd found: NO (tried pkgconfig and cmake)

../output-1/build/lxc-5.0.1/config/init/systemd/meson.build:11:4: ERROR: Dependency "systemd" not found, tried pkgconfig and cmake

Fixes:
 - http://autobuild.buildroot.org/results/495175c367084093bc01c40290736c738d876894



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>