Allow 2021 in LICENSE header.

Signed-off-by: Denton Gentry <[email protected]>

scripts/check_license_headers.sh

@@ -10,7 +10,7 @@
 check_file() {
     got=$1
 
-    for year in `seq 2019 2020`; do
+    for year in `seq 2019 2021`; do
         want=$(cat <<EOF
 // Copyright (c) $year Tailscale Inc & AUTHORS All rights reserved.
 // Use of this source code is governed by a BSD-style