main/git: add openssh as runtime dependency

The git tool can shell out to the ssh client binary for operations like "git push", and as such it needs the openssh package installed to function correctly.

I checked Fedora and they seem to depend on openssh in their git package. I imagine it's fairly rare for people to not have that package installed anyway, but I think it should be listed as a dependency. My first time contributing to Alpine, so my apologies if I've made a mistake somewhere.