Enable license header soundness check

Author: stmontgomery

.github/workflows/pull_request.yml

@@ -23,7 +23,6 @@ jobs:
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
     with:
       license_header_check_project_name: "Swift"
-      license_header_check_enabled: false
       docs_check_enabled: false
       unacceptable_language_check_enabled: false
       format_check_enabled: false