renderer_vulkan: assume X11 if not Windows/macOS after bf1d66b7

Render.Vulkan <Error> video_core/renderer_vulkan/renderer_vulkan.cpp:CreateInstance:131: Presentation not supported on this platform
Render.Vulkan <Error> video_core/renderer_vulkan/renderer_vulkan.cpp:CreateSurface:378: Presentation not supported on this platform
Core <Critical> core/core.cpp:Load:199: Failed to initialize system (Error 5)!