Advertise rollntime extension support

Thanks to raijin for testing!

util.c

@@ -339,6 +339,8 @@ json_t *json_rpc_call(CURL *curl, const char *url,
 
 	headers = curl_slist_append(headers,
 		"Content-type: application/json");
+	headers = curl_slist_append(headers,
+		"X-Mining-Extensions: rollntime");
 	headers = curl_slist_append(headers, len_hdr);
 	headers = curl_slist_append(headers, user_agent_hdr);
 	headers = curl_slist_append(headers, "Expect:"); /* disable Expect hdr*/