kernel-headers: mark broken for grsecurity kernels

Extracting headers from a grsecurity patched kernel triggers additional
build steps that require gcc plugins.  For this to work, we'd need to
add gmp, libmpfr, and libmpc to the build inputs as well as run `make
prepare` before installing the headers (lest the build fail due to
missing files).

Out-of-tree modules use kernel.dev and user space should use the Linux
API headers used to build libc, not headers extracted from random
kernels, so fixing this for grsecurity is pointless.

pkgs/os-specific/linux/kernel-headers/default.nix

@@ -1,5 +1,7 @@
 { stdenv, kernel, perl }:
 
+assert (!(kernel.features.grsecurity or false));
+
 let
   baseBuildFlags = [ "INSTALL_HDR_PATH=$(out)" "headers_install" ];
 in stdenv.mkDerivation {