D3D11: Fix warning that showed up in VS2019.
D3D11: Fix warning that showed up in VS2019.

Was related to the new D3D11on12 code.

Bug: angleproject:3986
Change-Id: I801b816c620a7307ee5a236856949866a8daf747
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1854720
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle