bridge-utils: fix a struct in6_addr reference

Bridge-utils uses kernel headers.  Linux 3.8 has a header,
include/uapi/linux/if_bridge.h that uses a struct in6_addr but
doesn't define it.  The trivial seeming fix of including the header
that does define it causes more problems.  The problem was discussed
on mailing lists in January 2013.  The final suggestion I found was
here:

  http://www.redhat.com/archives/libvir-list/2013-January/msg01253.html

This is intended to implement that suggestion.

Signed-off-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@35999 3c298f89-4303-0410-b956-a3cf2f4a3e73