Fix Windows CI build. (google#54)

The Windows SDK versions present on our CI options (GitHub Actions
hosted runners, AppVeyor) have headers that cause compilation warnings
when included with MSVC operating in modern C modes (C11+).

This PR disables the compilation warning caused by the Windows SDK
headers, so we can get Windows feedback from CI. The warnings can be
re-enabled when the Windows SDK used by our CI is upgraded to a version
that doesn't trigger warnings.