[Rspamd] Sign ARC inbonud, thanks to @Kraeutergarten

data/conf/rspamd/local.d/arc.conf

@@ -28,3 +28,5 @@ use_redis = true;
 key_prefix = "DKIM_PRIV_KEYS";
 # Selector map
 selector_prefix = "DKIM_SELECTORS";
+sign_inbound = true;
+use_domain_sign_inbound = "recipient";