vfio: selftests: Ensure libvfio output dirs are always created

vfio: selftests: Ensure libvfio output dirs are always created

Add an explicit dependency between the output object files and the
output directories that need to be created to hold those files. This
ensures that the output directories are always created.

Creating the output directories at parse time (current behavior) doesn't
support the scenario where someone does "make clean all". The
directories will be created during parsing, deleted during "clean" and
then not available for the "all" target.

Use an order-only prerequisite for the output directories, rather than a
normal prerequisite, to avoid unnecessary recompilations.

Fixes: 19faf6fd969c ("vfio: selftests: Add a helper library for VFIO selftests")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/kvm/20260610010314.DB8861F00893@smtp.kernel.org/
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20260611213945.3714421-1-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>