Just keep the definition of _WIN32_WINNT in windows.mk, which
claims it automatically derivates WINVER in some sdk header.

Change-Id: I0a83e91ffdc9e0fc847433a92a45424fbfcb189c
Reviewed-on: https://gerrit.libreoffice.org/61631Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>