layers: Tag internal helper functions as static
layers: Tag internal helper functions as static

Reduce the scope of internal helper functions by
declaring them static.

This is only changes lines related to VL-69,
rather than a full sweep of the code.

Change-Id: I3593e519d11066cd8631f56131de39e6e3b73f50

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