Deleting the following files and then reinstalling haproxy-devel worked for me:
/usr/local/pkg/haproxy.inc
/usr/local/pkg/haproxy_htmllist.inc
/usr/local/pkg/haproxy_rules.inc
/usr/local/pkg/haproxy_socketinfo.inc
/usr/local/pkg/haproxy_upgrade_config.inc
/usr/local/pkg/haproxy_utils.inc
/usr/local/pkg/haproxy_xmlrpcsyncclient.inc
/usr/local/pkg/pkg_haproxy_tabs.inc
I left the following files intact:
/usr/local/pkg/haproxy.xml
/usr/local/pkg/haproxy_rules.inc