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-repo-static-delta-core.c

@@ -1270,7 +1270,7 @@ ostree_repo_static_delta_reindex (OstreeRepo                 *repo,
 
   /* Protect against parallel prune operation */
   g_autoptr(OstreeRepoAutoLock) lock =
-    _ostree_repo_auto_lock_push (repo, OSTREE_REPO_LOCK_SHARED, cancellable, error);
+    ostree_repo_auto_lock_push (repo, OSTREE_REPO_LOCK_SHARED, cancellable, error);
   if (!lock)
     return FALSE;