Change `foreign import` for sdl3

vendor/sdl3/sdl3__foreign.odin

@@ -1,6 +1,7 @@
 package sdl3
 
-@(export)
-foreign import lib {
-	"SDL3.lib",
+when ODIN_OS == .Windows {
+	@(export) foreign import lib { "SDL3.lib" }
+} else {
+	@(export) foreign import lib { "system:SDL3" }
 }