www/otrs: Add missing sysutils/p5-Schedule-Cron-Events dependency

PR: 248577
Submitted by: vvv <work+freebsd@vvv.kiev.ua>
Approved by: m.tsatsenko@gmail.com (maintainer)
MFH: 2021Q2 (runtime fix)

(cherry picked from commit e0154347055c7dc1656a097280ff968037c9bb98)