pluggable.php warning

Warning: Cannot modify header information - headers already sent by (output started at /home/leader/public_html/wp-content/plugins/custom-fields-rss/custom-fields-rss.php:15) in /home/leader/public_html/wp-includes/pluggable.php on line 1121

Getting this error in feed adress: http://www.leadergamer.com.tr/feed/

How can I solve this?

https://wordpress.org/plugins/custom-fields-rss/