package/swupdate: add optional systemd dependency

package/swupdate: add optional systemd dependency

When the swupdate SYSTEMD option is enabled, systemd needs to be built
before swupdate, otherwise the build fails with:

core/notifier.c:27:10: fatal error: systemd/sd-daemon.h: No such file or directory
   27 | #include <systemd/sd-daemon.h>
      |          ^~~~~~~~~~~~~~~~~~~~~

Of course, it remains up to the user to make sure that the systemd
package is enabled when systemd support is enabled in the swupdate
configuration.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>