www/gatling: Mark BROKEN

Gatling hasn't built for months and there's also a new version available.
Various errors but most common seems to be:
"ld: error: duplicate symbol: first_deferred"

Reference: https://portsfallout.com/fallout?port=www%2Fgatling%24&page=1

PR: 259180
Reported by: pkg-fallout
Approved by: port maintainer timeout (2+ weeks), arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D32769