Add Daniel Austin to contributors for net-mgmt/cacti

He is also the maintainer of these ports:
emulators/atari800
graphics/gocr
irc/miau
irc/undernet-ircu
mail/milter-greylist-devel
mail/opendkim
net/rtg
net-mgmt/klg
sysutils/sas2ircu
x11-wm/ccsm
x11-wm/compiz
x11-wm/compiz-fusion
x11-wm/compiz-plugins-extra
x11-wm/compiz-plugins-main
x11-wm/compiz-plugins-unsupported
x11-wm/compizconfig-backend-gconf
x11-wm/compizconfig-python
x11-wm/emerald
x11-wm/libcompizconfig
x11-wm/simple-ccsm

PR: 219854
Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D11123