Enable license header check in pull_request.yml

Author: MaxDesiatov

.github/workflows/pull_request.yml

@@ -42,6 +42,6 @@ jobs:
       license_header_check_project_name: "Swift"
       api_breakage_check_enabled: false
       unacceptable_language_check_enabled: true
-      license_header_check_enabled: false
+      license_header_check_enabled: true
       broken_symlink_check_enabled: true
       format_check_enabled: false