NSIS: check Windows version before running uninstaller

Change-Id: Icc8d725bff484c48d14e0a9208c545ca512a9768
Reviewed-on: https://code.wireshark.org/review/3533
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Christopher Maynard <Christopher.Maynard@gtech.com>
Petri-Dish: Christopher Maynard <Christopher.Maynard@gtech.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>