Revert "Unify foreign import for vendor:sdl2"

This reverts commit b2b88f1d99.

vendor/sdl2/sdl_vulkan.odin

@@ -3,6 +3,12 @@ package sdl2
 import "core:c"
 import vk "vendor:vulkan"
 
+when ODIN_OS == .Windows {
+	foreign import lib "SDL2.lib"
+} else {
+	foreign import lib "system:SDL2"
+}
+
 VkInstance   :: vk.Instance
 VkSurfaceKHR :: vk.SurfaceKHR