layers: Fix signed/unsigned comparison warnings in thread_check.h
layers: Fix signed/unsigned comparison warnings in thread_check.h

Change-Id: I3a5e8e64deed53b3817be1fe8e1e2b32c08691a7

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