ipc, rwcancel: compile on js/wasm

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipc/uapi_js.go

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package ipc
+
+// Made up sentinel error codes for the js/wasm platform.
+const (
+	IpcErrorIO        = 1
+	IpcErrorInvalid   = 2
+	IpcErrorPortInUse = 3
+	IpcErrorUnknown   = 4
+	IpcErrorProtocol  = 5
+)