dockerfiles/debian-win64-cross: Download WHPX MinGW headers

To compile-test the WHPX accelerator, we need to download these system
headers first (they are unfortunately not part of any released and
packaged MinGW toolchain yet).

Idea taken from another patch by Stefan Weil.

Message-Id: <20200804170055.2851-12-thuth@redhat.com>
Message-Id: <20200823111757.72002-6-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>