Oh damn. I pinned the postgresql and postgresql-libs packages so that they would not "break" on a system upgrade and this was the culprit. After up...

Do you have any other (especially third party) PHP extensions enabled?

I am not able to reproduce the issue:...

I am rather certain this is a packaging issue because I am able to build, install and load the module in a docker container using the php docker image...

You're right, I wanted to check something first and wanted to reply there as well, but I see you were quicker. Thanks for linking :)

I think this was also reported on the forums: https://bbs.archlinux.org/viewtopic.php?id=303484