lib: Fix a bad call to g_file_get_child

lib: Fix a bad call to g_file_get_child

In Glib, since commit 3a6e8bc8876e149c36b6b14c6a25a718edb581ed,
`g_file_get_child` does not accept absolute path as paramater anymore.

The broken assertion was encountered during `ostree admin deploy`
command for the checkout of subpath `etc`.

Example of error log:
```
(ostree admin deploy:1640): GLib-GIO-CRITICAL **: 03:42:00.570: g_file_get_child: assertion '!g_path_is_absolute (name)' failed

(ostree admin deploy:1640): GLib-GIO-CRITICAL **: 03:42:00.570: g_file_query_info: assertion 'G_IS_FILE (file)' failed
**
OSTree:ERROR:src/ostree/ot-main.c:232:ostree_run: assertion failed: (success || error)
Bail out! OSTree:ERROR:src/ostree/ot-main.c:232:ostree_run: assertion failed: (success || error)
```