kcoreaddons_target_static_plugins: Use private linking for plugin registration file
In case the plugins are bundled in a lib, all resulting executables would be forced to compile this file. In the KWin case this was intended for, linking the static effect plugins to the shared test lib work too.
This also helps to better isolate the plugin registrations.
In case the plugins are only bundled in a single executable, like KMyMoney, there will it will work exectly as before.
See plasma/kwin@52931921 how this would be utilized in KWin.