cube: silence -DNDEBUG warning for KHR_display after 27c21248221f
cube: silence -DNDEBUG warning for KHR_display after 27c21248221f

cube/cube.c:3122:14: error: variable 'result' set but not used [-Werror,-Wunused-but-set-variable]
    VkResult result;
             ^

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools