wgengine/router: make runner.go linux-only for now.

Otherwise, staticcheck complains that these functions are unused
and unexported on macOS.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/router/runner.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux
+
 package router
 
 import (