libnetwork/iptables: un-export FirewalldInit
It's only used internally, and it was last used in commit:
https://github.com/moby/libnetwork/commit/0220b06cd6e0048adef13aba5cba3fea8be7fede
But moved into the iptables package in this commit:
https://github.com/moby/libnetwork/commit/998f3ce22c641b33f062d111f043e92495e3804c
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>