Update header-checker config (GoogleCloudPlatform#5358)

* Update header-checker config

This will unblock GoogleCloudPlatform#5352

* also exclude __init__.py

Author: leahecole

.github/header-checker-lint.yml

@@ -1,6 +1,6 @@
 {"allowedCopyrightHolders": ["Google LLC"],
  "allowedLicenses": ["Apache-2.0", "MIT", "BSD-3"],
- "ignoreFiles": ["**/requirements.txt", "**/requirements-test.txt"],
+ "ignoreFiles": ["**/requirements.txt", "**/requirements-test.txt", "**/requirements-composer.txt", "__init__.py"],
  "sourceFileExtensions": [
  	"ts", 
  	"js",