Exclude airflowignore_all from the license check

It's a plain .airflowignore-style pattern file, not source, same as
the existing .airflowignore entry.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jason810496

.rat-excludes

@@ -215,6 +215,7 @@ www-hash.txt
 **/.prettierrc
 **/.airflowignore
 **/.airflowignore_glob
+**/airflowignore_all
 
 
 # Vendor includes