x11-wm/i3: fix build with many threads

i3 needs MAKE_JOBS_UNSAFE=yes to build.

Errors:
/wrkdirs/usr/ports/x11-wm/i3/work/i3-4.16/../i3-4.16/i3-config-wizard/main.c:109:10: fatal error: GENERATED_config_enums.h: No such file or directory
#include "GENERATED_config_enums.h"

^~~~~~~~~~~~~~~~~~~~~~~~~~

compilation terminated.

PR: 237890
Approved by: tcberner (mentor), bapt (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D20438