KWin decorations KCM: Fix broken AdoptionCommand for KNewStuff
We need CMAKE_INSTALL_FULL_LIBEXECDIR, otherwise lib/x86_64-linux-gnu/libexec/kwin-applywindowdecoration
will be the resulting path, which does not contains the cmake install prefix.
Consequently KNS would fail to find it at runtime.
BUG: 447284 FIXED-IN: 5.24.0