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: https://github.com/FiloSottile/yubikey-agent/issues/137

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

···
       ];
     };
 
+    # 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"