add trailing newline to subgid due to shadow-utils 4.19.0 regression

Fedora 44 come with shadow-utils 4.19.0, This is a POSIX compliance fix - text files need end with a newline.
1. https://github.com/containers/podman-machine-os/issues/226
2. https://github.com/shadow-maint/shadow/issues/1581

Signed-off-by: Tim Zhou <[email protected]>