nixos-19.03: fix hardened kernels

[edwintorok]

On NixOS 19.03 I get:

Error: modDirVersion 4.19.34 specified in the Nix expression is wrong, it should be: 4.19.34-hardened

Cherry-pick 922c07d from master to fix it.

Motivation for this change

I was using the 19.03 beta (or so I thought, apparently I was actually using the unstable channel) and tried to switch to the 19.03 release version yesterday, which failed.
After cherry-picking the above commit I was able to successfully switch.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[teto]

there was a PR to automatically set this. If you are interested #31610

[joachifm]

@GrahamcOfBorg build linux_hardened

[joachifm]

Thank you. As an aside, cherry-pick commits should be created with cherry-pick -x to retain a pointer to the original in the commit message.