tethering: Fix build with kernel headers 3.8+

tethering: Fix build with kernel headers 3.8+

Highly inspired by:
    https://git.kernel.org/cgit/linux/kernel/git/shemminger/bridge-utils.git/commit/?id=5eebb7f9288b7881ffb929b1fd494fe3ac3be27d

As Russel puts it in his commit message for bridge-utils:

    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.

This changeset transplants this trivial fix to connman.