Hi reviewers,

k3b doesn't build anymore on current master due to a mix-up of CMake 
variables.  This patch fixes that.

It's probably a temporary issue because latest versions of k3b have 
updated CMakeLists, but updating to a newer k3b also requires 
updating all KDE Frameworks to version 5.88.  So I hope we can 
include the present fix for now.

It's been a while, hope this patch satisfies the requirements :-)

Sincerely,

Thomas