pocketfft's host (gitlab.mpcdf.mpg.de) is down, breaking HEIR builds from scratch

Probably won't affect most of us already actively working on HEIR since we'll have the repo already fetched locally, but gitlab.mpcdf.mpg.de, which hosts our (transitive?) dependency "pocketfft" is down. It looks like there's a GitHub mirror (https://github.com/mreineck/pocketfft), if that's legit, maybe we should transition to depending on that?