platform: Fix build on windows

experimental/libbox/service.go

@@ -79,7 +79,7 @@ func (w *platformInterfaceWrapper) OpenTun(options tun.Options, platformOptions
 	if err != nil {
 		return nil, err
 	}
-	dupFd, err := syscall.Dup(int(tunFd))
+	dupFd, err := dup(int(tunFd))
 	if err != nil {
 		return nil, E.Cause(err, "dup tun file descriptor")
 	}

experimental/libbox/service_other.go

@@ -0,0 +1,9 @@
+//go:build !windows
+
+package libbox
+
+import "syscall"
+
+func dup(fd int) (nfd int, err error) {
+	return syscall.Dup(fd)
+}

experimental/libbox/service_windows.go

@@ -0,0 +1,7 @@
+package libbox
+
+import "os"
+
+func dup(fd int) (nfd int, err error) {
+	return 0, os.ErrInvalid
+}