radvd: fix building for x32

Fix is from upstream Gentoo.

BUG=chromium-os:36866
TEST=`emerge-x32-generic radvd` works

Change-Id: Id95f875a9e0bc513bda136d6c62c90fdaf565835
Reviewed-on: https://gerrit.chromium.org/gerrit/45423
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
2 files changed