nixos/yubikey-agent: Add dependency to pcsd.service

Issue: after installing, running `yubikey-agent -setup` produces the
following error:
```
nixOS: Failed to connect to the YubiKey: connecting to pscs: the Smart
card resource manager is not running
```

More on this issue: FiloSottile/yubikey-agent#137

nixos/modules/services/security/yubikey-agent.nix

@@ -57,6 +57,9 @@ in
  ];
  };
 
+ # Yubikey-agent expects pcsd to be running in order to function.
+ services.pcscd.enable = true;
+
  environment.extraInit = ''
  if [ -z "$SSH_AUTH_SOCK" -a -n "$XDG_RUNTIME_DIR" ]; then
  export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/yubikey-agent/yubikey-agent.sock"