Win: Add pre-built OpenSSL as dep

This adds pre-built OpenSSL binaries to the Windows 3rd-party deps. The
binaries are provided by the curl project [1] [2] using a reproducible
process with publicly-viewable build logs. It should be reputable enough
for our usage.

[1]: https://curl.haxx.se/windows/
[2]: https://github.com/curl/curl-for-win

Differential Revision: https://phabricator.kde.org/D20206