libglnx porting: Use glnx_set_error_from_errno

⚠️ There is a notable spiked pit trap here around
`posix_fallocate()` and `errno`.  This has bit other projects,
see e.g.
https://github.com/systemd/systemd/commit/7bb87460e691d30c1a7fd23a1a8240776957e05f

Otherwise the port was straightforward.

src/libostree/ostree-repo-pull.c

@@ -841,7 +841,7 @@ meta_fetch_on_complete (GObject           *object,
   fd = openat (_ostree_fetcher_get_dfd (fetcher), temp_path, O_RDONLY | O_CLOEXEC);
   if (fd == -1)
     {
-      gs_set_error_from_errno (error, errno);
+      glnx_set_error_from_errno (error);
       goto out;
     }