Backout the vulkan change in d449dea10fc8 breaks Mir builds
diff --git a/src/video/SDL_vulkan_internal.h b/src/video/SDL_vulkan_internal.h
index 261fc9c..cdf464e 100644
--- a/src/video/SDL_vulkan_internal.h
+++ b/src/video/SDL_vulkan_internal.h
@@ -38,6 +38,9 @@
#if SDL_VIDEO_DRIVER_COCOA
#define VK_USE_PLATFORM_MACOS_MVK
#endif
+#if SDL_VIDEO_DRIVER_MIR
+#define VK_USE_PLATFORM_MIR_KHR
+#endif
#if SDL_VIDEO_DRIVER_UIKIT
#define VK_USE_PLATFORM_IOS_MVK
#endif