batman-adv: mark as broken on -grsec

Looks to be incompatible with the PaX constification plugin:

> /tmp/nix-build-batman-adv-2016.2-4.7.2.drv-0/batman-adv-2016.2/net/batman-adv/soft-interface.c:1065:22:
error: constified variable 'batadv_link_ops' placed into writable
section ".data..read_mostly"
 struct rtnl_link_ops batadv_link_ops __read_mostly = {

https://hydra.nixos.org/build/39312033/log/raw