Obtain a secondary school teaching certificate?
In the big picture, having a secondary school teaching certificate probably won't make a difference in the hiring process for lecturer positions at the university level. Your teaching experience is what will count (to some extent--really, teaching at the collegiate level will be more important), and you will have that to put on your CV. I highlighted my high school teaching experience (and degree, certificate, etc.) on my CV when I applied to teaching positions, and in the interviews I had it was clear that it helped me land the interviews.

That said, if the certificate is something that will give you a fall-back plan if the PhD doesn't work out (i.e., you can transition directly back into the secondary teaching more easily), it might not be a bad idea to get it.

Either way, having the certificate won't hurt you when applying for future collegiate teaching jobs, and if you have the time now it might not be a bad idea to go ahead and knock it out.