net/ntp: add build_depends autogen

[concussious.bugzilla_runbox.com]

Summary

Previously, the port would not build complaining about no autogen. With this patch, the port builds and installs.

I have no idea what I'm doing, this is my first time trying to build_depends, so please exercise extreme caution.

This gets me a little further than I was getting on https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=282185

Probably unrelated, when I did this, the port built, but then my system (current built yesterday) hung completely unresponsive...

Test Plan

Look it over very carefully.
Then patch, build, and install.

[cy]

LGTM

[cy]

Next time, can you please write what you'd like for a commit log entry. I'm not comfortable writing commit log entries for other people.

[concussious.bugzilla_runbox.com]

Absolutely, thank you.