Add selinux-label-version to the org.osbuild.ostree.commit stage

This is a feature that was added in rpm-ostree 2023.10 and is needed
for the new transient /etc feature to work. What it does is change the
labeling of /usr/etc to match those of /etc, so that /usr/etc can be used
directly as a bind-mount or an overlay mount when mounted on /etc.

See https://github.com/coreos/rpm-ostree/pull/4640 for details.

osbuild/util/ostree.py

@@ -57,6 +57,7 @@ class Treefile:
       - boot-location: postprocess
       - etc-group-members: postprocess
       - machineid-compat
+      - selinux-label-version: commit
 
     NB: 'ref' and 'repos' are mandatory and must be
     present, even if they are not used in the given
@@ -71,6 +72,7 @@ class Treefile:
         "etc-group-members": Param(List[str]),
         "machineid-compat": Param(bool),
         "initramfs-args": Param(List[str]),
+        "selinux-label-version": Param(int),
     }
 
     def __init__(self):