gitian: add libz-dev dependency package for linux boost

Boost iostreams was picking up libz-dev in VirtualBox, as the recommended
way to build is now to make a VM with all dependency packages installed.

This caused a divergence between KVM/LXC build and VirtualBox
build results.

Fix this in the simplest possible way: add the libz-dev package.
Merge pull request #3728

6c0276a gitian: add libz-dev dependency package for linux boost (Wladimir J. van der Laan)