Make both wrapper structs extern

lib/minifb/src/minifb.zig

@@ -24,7 +24,7 @@ pub const KeyMod = packed struct {
 };
 
 pub fn Window(comptime TUserData: type) type {
-    return packed struct {
+    return extern struct {
         cwin: *c.mfb_window,
 
         pub const UpdateError = error{