cube: Make Volk requirement explicit

[lunarpapillo]

See also:

Cannot build DEMOS.sln
https://gitlab.khronos.org/vulkan/Vulkan-SDK-Packaging/-/issues/1417

Volk requires that VK_NO_PROTOTYPES be defined before vulkan.h or vulkan.hpp is included. Currently, the various flavors of vkcube hide this definition in the cube/CMakeLists.txt file, which can confuse users who may copy the source for their own use, and may require investigation to figure out why it doesn't "just work".

This change makes the #define explicit in the cube.c and cube.cpp source files, which should both be clearer and be more similar to how most applications use Volk.