Package request: Vulkan validation layers

https://github.com/KhronosGroup/Vulkan-ValidationLayers

The validation layers are needed to use the experimental Vulkan renderer introduced in wlroots 0.15.

$ WLR_RENDERER=vulkan sway
[...]
00:00:00.002 [ERROR] [wlr] [render/vulkan/vulkan.c:208] Could not create instance: ERROR_LAYER_NOT_PRESENT (-6)
00:00:00.002 [ERROR] [wlr] [render/vulkan/renderer.c:1480] creating vulkan instance for renderer failed
00:00:00.002 [ERROR] [sway/server.c:79] Failed to create renderer