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.

contrib/gitian-descriptors/boost-linux.yml

@@ -13,6 +13,7 @@ packages:
 - "faketime"
 - "bsdmainutils"
 - "zip"
+- "libz-dev"
 reference_datetime: "2011-01-30 00:00:00"
 remotes: []
 files: