device: allow compiling with Go 1.15

Until we depend on Go 1.16 (which isn't released yet), alias our own
variable to the private member of the net package. This will allow an
easy find replace to make this go away when we eventually switch to
1.16.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/net_err_closed.go

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package conn
+
+import _ "unsafe"
+
+//TODO: replace this with net.ErrClosed for Go 1.16
+
+//go:linkname NetErrClosed internal/poll.ErrNetClosing
+var NetErrClosed error