lttng-modules: disable on 3.12 kernel

pkgs/os-specific/linux/lttng-modules/default.nix

···
 { stdenv, fetchurl, kernel }:
 
 assert stdenv.lib.versionAtLeast kernel.version "3.4";  # fails on 3.2
+assert builtins.substring 0 4 kernel.version != "3.12";
 
 stdenv.mkDerivation rec {
   pname = "lttng-modules-${version}";