fix: add license header to pre-commit

Author: miraleung

.githooks/pre-commit

@@ -1,5 +1,19 @@
 #!/bin/sh
 
+# Copyright 2021 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
+#
+#     https://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.
+
 # Pre-commit Git checks.
 # Set up:
 #  ln -s .githooks/pre-commit .git/hooks/pre-commit