Fix Windows build.

When using CMake to build with MSVC, MSVC complains about unreachable
code in the <xtree> header. This incantation silences that.

Change-Id: I5fc5305dc816a009a4c59501b212fd11e290637d
Reviewed-on: https://boringssl-review.googlesource.com/5552
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>