Add gitignore

Created by: paigeweber13

Hey Tom, it's been a while! Hope you're doing well :) Also, I'm changing my name from "Riley" to "Paige", sorry for the confusion.

I don't know if this aligns with your vision for this repo, but I frequently found myself wishing I had a gitignore while working on this repository, as a clean make generates hundreds of files that are picked up by git. So I created one that just ignores everything generated by Make

If this fits your goals I'd love to see it added into likwid.