cmake: Change name of helper-file cmake target
cmake: Change name of helper-file cmake target

Cmake won't tolerate multiple build targets withing a project.
Generate_helper_files has been renamed to resolve conflicts with
other repos.

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