Revert "Unify foreign import for vendor:sdl2"

This reverts commit b2b88f1d99.

vendor/sdl2/sdl_messagebox.odin

@@ -2,6 +2,12 @@ package sdl2
 
 import "core:c"
 
+when ODIN_OS == .Windows {
+	foreign import lib "SDL2.lib"
+} else {
+	foreign import lib "system:SDL2"
+}
+
 MessageBoxFlag :: enum u32 {
 	_ = 0,
 	ERROR                 = 4,   /**< error dialog */