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

pkgs/os-specific/linux/batman-adv/default.nix

@@ -24,5 +24,6 @@ stdenv.mkDerivation rec {
     license = stdenv.lib.licenses.gpl2;
     maintainers = with stdenv.lib.maintainers; [ viric fpletz ];
     platforms = with stdenv.lib.platforms; linux;
+    broken = (kernel.features.grsecurity or false);
   };
 }