d3d11: avoid exit-time-destructor warning Applies to use_custom_libcxx=false on Windows builds. On MSVC's C++ runtime, std::mutex has a destructor. Bug: angleproject:8327 Change-Id: I6118753569ce0529015e82673571c961a38f30e6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4819793 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Auto-Submit: Steven Noonan <steven@uplinklabs.net> Reviewed-by: Geoff Lang <geofflang@chromium.org>