system: unbreak build for darwin

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

pkg/system/mknod_linux.go → pkg/system/mknod_unix.go

@@ -1,3 +1,6 @@
+//go:build !freebsd && !windows
+// +build !freebsd,!windows
+
 package system // import "github.com/docker/docker/pkg/system"
 
 import (