nixos: condition sysctl.kptr_restrict on features.grsecurity

Conditioning `sysctl.kptr_restrict` on `features.grsecurity` supports
any grsecurity enabled kernel without having to enable the grsecurity
module.

nixos/modules/config/sysctl.nix

···
     #
     # Removed under grsecurity.
     boot.kernel.sysctl."kernel.kptr_restrict" =
-      if config.security.grsecurity.enable then null else 1;
+      if (config.boot.kernelPackages.kernel.features.grsecurity or false) then null else 1;
   };
 }