lxc-auto: made init script compatible with image builder

[fantom-x]

Maintainer: @ratkaj @BKPepe

Fixes this message from image builder:

Finalizing root filesystem...
/build/openwrt-imagebuilder/build_dir/target-x86_64_musl/root-x86/etc/init.d/lxc-auto: line 3: /lib/functions.sh: No such file or directory
Enabling boot
Enabling cgroupfs-mount
Enabling collectd

Closes https://github.com/openwrt/packages/issues/19292

Signed-off-by: Marc Benoit [email protected]