layers: Avoid warnings in vk_layer_logging.h
layers: Avoid warnings in vk_layer_logging.h

This file was often included in files where only a subset of the
functions were called, resulting in -Wunused_function warnings from
GCC. Made these functions inline to avoid the warnings.

Change-Id: Iaa3765ee51a290f0dd82d1a6a91116b6e1eec2c1

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