message: Add std:: to nullptr_t
  Fixes compiler error on Linux about bare nullptr_t not existing.
diff: @@ -174,7 +174,7 @@ namespace dawn_native { namespace vulkan {
  // Redefine VK_NULL_HANDLE for better type safety where possible.
  #undef VK_NULL_HANDLE
  #if defined(DAWN_PLATFORM_64_BIT)
  -static constexpr nullptr_t VK_NULL_HANDLE = nullptr;
  +static constexpr std::nullptr_t VK_NULL_HANDLE = nullptr;
  #elif defined(DAWN_PLATFORM_32_BIT)
  static constexpr uint64_t VK_NULL_HANDLE = 0;
  #else
  