wgengine: stop importing flowtrack when unused

Updates #12614

Change-Id: I42b5c4d623d356af4bee5bbdabaaf0f6822f2bf4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/pendopen_omit.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_debug
+
+package wgengine
+
+import (
+	"tailscale.com/net/packet"
+	"tailscale.com/net/tstun"
+	"tailscale.com/wgengine/filter"
+)
+
+type flowtrackTuple = struct{}
+
+type pendingOpenFlow struct{}
+
+func (*userspaceEngine) trackOpenPreFilterIn(pp *packet.Parsed, t *tstun.Wrapper) (res filter.Response) {
+	panic("unreachable")
+}
+
+func (*userspaceEngine) trackOpenPostFilterOut(pp *packet.Parsed, t *tstun.Wrapper) (res filter.Response) {
+	panic("unreachable")
+}