layers: Fix VkBool32 return in vk-layer-generate.py.
layers: Fix VkBool32 return in vk-layer-generate.py.

This regressed in f4542e618, but is only visible when defining
VK_USE_PLATFORM_WIN32_KHR, so might have been missed. Fixes a
warning in MS 2015, as well as an incorrect return value.

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