gitian: add libz-dev dependency package for linux boost
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.

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