grsecurity module: systemd-nspawn requires cap_sys_admin

As with 9ca3504a798291fbd7c49fcfeec8b64daa2022ad

Closes https://github.com/NixOS/nixpkgs/issues/17714

nixos/modules/security/grsecurity.nix

···
       "kernel.grsecurity.chroot_deny_chmod" = mkForce 0;
       "kernel.grsecurity.chroot_deny_mount" = mkForce 0;
       "kernel.grsecurity.chroot_restrict_nice" = mkForce 0;
+      "kernel.grsecurity.chroot_caps" = mkForce 0;
     };
 
     assertions = [