This part was wrongly inheriting from nginx, but it should inherit from
nginx-common instead, so that it is not shared, because as it installs from a
git clone we can not share it.

It would be possible to make it shared by downloading a git archive and using
a dummy slapos.recipe.cmmi recipe for nginx-push-stream-module parts, so that
it is also shared, but since nothing depends on nginx and it is small, that
does not seem to be worth it.