[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

Not sure if this is the best way to add a dependency to a service tho - open for input.

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

@@ -56,6 +56,9 @@ in
           "graphical-session.target")
       ];
     };
+
+    # 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