grub: disable stackprotector hardening

pkgs/tools/misc/grub/default.nix

···
   # autoreconfHook required for the splashimage patch.
   buildInputs = [ autoreconfHook texinfo ];
 
+  hardening_stackprotector = false;
+
   prePatch = ''
     unpackFile $gentooPatches
     rm patch/400_all_grub-0.97-reiser4-20050808-gentoo.patch