nixos/dhparams: mark service ${name} requires dhparams-gen-${name}

This can prevent someone delete the generated file in `/var/lib/dhparams/${name}.pem`
and the `${name}.service` will still be happy to start
without starting `dhparams-gen-${name}` to re-generate the deleted file.

nixos/modules/security/dhparams.nix

@@ -194,6 +194,7 @@ in
         description = "Generate Diffie-Hellman Parameters for ${name}";
         after = [ "dhparams-init.service" ];
         before = [ "${name}.service" ];
+        requiredBy = [ "${name}.service" ];
         wantedBy = [ "multi-user.target" ];
         unitConfig.ConditionPathExists = "!${path}";
         serviceConfig.Type = "oneshot";