Add SELinux support

The trees as shipped come with /usr/etc, which should just be labeled
as usr_t.  When we do a deployment, we need to relabel the copies of
the files we're making in /etc.

SELinux support is compile and runtime optional.

src/libostree/ostree-repo.h

@@ -307,6 +307,7 @@ typedef GVariant *(*OstreeRepoCommitModifierXattrCallback) (OstreeRepo     *repo
 
 void ostree_repo_commit_modifier_set_xattr_callback (OstreeRepoCommitModifier              *modifier,
                                                      OstreeRepoCommitModifierXattrCallback  callback,
+                                                     GDestroyNotify                         destroy,
                                                      gpointer                               user_data);
 
 OstreeRepoCommitModifier *ostree_repo_commit_modifier_ref (OstreeRepoCommitModifier *modifier);