repo: Make locking APIs public

Doing anything even somewhat sophisticated requires this;
turns out our own `ostree prune` CLI wants this, e.g.
https://github.com/ostreedev/ostree/issues/2337

Closes: https://github.com/ostreedev/ostree/issues/2286

src/libostree/ostree-sysroot-cleanup.c

@@ -505,7 +505,7 @@ ostree_sysroot_cleanup_prune_repo (OstreeSysroot          *sysroot,
    * the prune.
    */
   g_autoptr(OstreeRepoAutoLock) lock =
-    _ostree_repo_auto_lock_push (repo, OSTREE_REPO_LOCK_EXCLUSIVE, cancellable, error);
+    ostree_repo_auto_lock_push (repo, OSTREE_REPO_LOCK_EXCLUSIVE, cancellable, error);
   if (!lock)
     return FALSE;