bibata-cursors: fix installation, don't generate unused w32 artifacts
This fixes issue where each theme installed the entire
build directory, not just the final generated 'out/X11' files.
(which was wasteful but also meant they didn't work
as the right files were two directories deeper)
Avoiding the w32 saves some time and sapce,
and more is "saved" by not shipping non-cursor build intermediates.
Sorry for the bad update, folks!