security/plasma5-kscreenlocker: add explicit dependency on qdbus

It's a requirement of the included ck-unlock-session script.
The binary found by the cmake configure to check for its availability
found the 'wrapper' installed by misc/qtchooser, which was present
always (in short: it was broken). An upcoming change will change the
behaviour of qtchooser not to install links if the binaries are not
present. This would lead ot ck-unlock-session no longer being installed
as the dependency is now (also for the cmake configure) missing.