package/iptables: fix build on musl

Fixes:
http://autobuild.buildroot.net/results/68b/68bf8830f87ab32f4fd650c9dd4a8c80592983a9/
http://autobuild.buildroot.net/results/d9b/d9bb677fb5510adf05ffb4afbc63ead83d7e18dd/
http://autobuild.buildroot.net/results/4b8/4b80b6d0131b624301a5cfcd5541897f411043c9/



.. and others.

This patch includes two backported patches from iptables git, plus one
pending patch not integrated yet.

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>