msys-gzip: Upgrade to 1.6

This version supports deterministic compression/decompression and is therefore a step into the direction of deterministic builds.

Buildbot: https://dscho.cloudapp.net/job/sdk-build-packages/14
This is what the default src_install() also does, so it should go away to simplify the file (see c760f24fe11aaf6dea0d27c1f397d378f35f0388).