scripts: remove special case for _strings.go files in check license headers

And add a license header for derp/dropreason_string.go to make it happy.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

scripts/check_license_headers.sh

@@ -38,9 +38,6 @@ for file in $(find $1 -name '*.go' -not -path '*/.git/*'); do
         $1/wgengine/router/ifconfig_windows.go)
             # WireGuard copyright.
         ;;
-		*_string.go)
-			# Generated file from go:generate stringer
-		;;
         *)
             header="$(head -3 $file)"
             if ! check_file "$header"; then