liblogging: Depend on systemd

This package depends on systemd because in the configure phase it
searches for a file installed by systemd. We add systemd to it's
dependencies to ensure that is built before this package.

Fixes:
   http://autobuild.buildroot.net/results/32c/32c636020600aa3f378d326c84fac82eb1fb2871/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>