Backout the vulkan change in d449dea10fc8 breaks Mir builds

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