Update .gitignore file

Added 7z, zip and gz archive types which are common to packaging related
tasks. Added also:

packaging-tools/config
packaging-tools/build_artifacts

These are temporary directories created by the packaging scripts.

Change-Id: I8387c338a3c33fe8fd9be91a63317007131d3bc1
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>