Edit the file /etc/logrotate.d/syslog. Find the first line, which should look like this (wrapped for clarity):

/var/log/messages /var/log/secure /var/log/maillog /var/log/spooler 
/var/log/boot.log /var/log/cron {

Edit this line so that it contains a one-space-separated listing of each log file referenced in /etc/syslog.conf