Add fallback to the absolute path to kscreenlocker_greet

KLibexec doesn't work on usr-merged distros as libraries can be loaded
through a different path (/lib*) than they are installed to (/usr/lib*).

BUG: 452817
2 jobs for !71 with work/fvogt/bug452817 in 2 minutes and 14 seconds (queued for 8 seconds)
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