This fixes a build issue when building with libc++ on Windows.

Bug: chromium:801780
Change-Id: I8fa84cefa272c2f3b096b2aada793192a47cc0e3
Reviewed-on: https://swiftshader-review.googlesource.com/16468Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Peter Collingbourne <pcc@google.com>