iproute2: drop unneeded Werror handling

Upstream has dropped -Werror since the 3.12 release, so there's no point in
us sed'ing it away:

https://git.kernel.org/cgit/linux/kernel/git/shemminger/iproute2.git/commit/?id=3140e9a3a343d37a8d828ee27e16ad9f3f9ab389



Reported-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>