[Minor] Register DKIM sign symbol in metric

src/plugins/dkim_check.c

@@ -438,6 +438,10 @@ dkim_module_config (struct rspamd_config *cfg)
 								"DKIM_SIGN", dkim_module_ctx->symbol_reject);
 					}
 
+					rspamd_config_add_metric_symbol (cfg, DEFAULT_METRIC,
+							"DKIM_SIGN", 0.0, "DKIM signature fake symbol",
+							"dkim", RSPAMD_SYMBOL_FLAG_IGNORE, 1);
+
 				}
 				else {
 					msg_err_config ("lua script must return "