wgengine/router/dns: appease staticcheck (again)

wgengine/router/dns/manager.go

@@ -10,6 +10,12 @@ import (
 	"tailscale.com/types/logger"
 )
 
+// We use file-ignore below instead of ignore because on some platforms,
+// the lint exception is necessary and on others it is not,
+// and plain ignore complains if the exception is unnecessary.
+
+//lint:file-ignore U1000 reconfigTimeout is used on some platforms but not others
+
 // reconfigTimeout is the time interval within which Manager.{Up,Down} should complete.
 //
 // This is particularly useful because certain conditions can cause indefinite hangs