Include the D3D11 headers on Windows when OpenGL is enabled. The OpenGL backend on Windows uses D3D11 to implement several interop extensions. Without this change, there are compile failures when the D3D11 backend is disabled. BUG=angleproject:2276 Change-Id: I69ad524ca84aa0aa399cfc0b601315f3ee65ed16 Reviewed-on: https://chromium-review.googlesource.com/809186 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>