vulkan: assume secure_getenv(). Vulkan's cmake files check for the availability of secure_getenv() but we just assume it exists. BUG=chromium:717558 Change-Id: I22054c3967f543d163dde37b9a6b13e896da7a50 Reviewed-on: https://chromium-review.googlesource.com/494126 Commit-Queue: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>