Added missing license header

Makefile

@@ -1,3 +1,9 @@
+build:
+	go build ./cmd/yamlfmt
+
+install:
+	go install ./cmd/yamlfmt
+
 install_tools:
 	go install github.com/google/addlicense@latest
 

helpers.go

@@ -1,3 +1,17 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package yamlfmt
 
 import (