package/systemd: select SELinux modules

Select the systemd and udev SELinux modules so that they will be
compiled in the refpolicy. This way, if an SELinux policy is generated,
Systemd will be supported.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>