Add `ext/` folder to .gitignore

This folder is populated in various build processes (civibuild's
`drupal-demo`, `wp-demo`, etc; as well as `distmaker`), and sometimes
I like to use it for personal things. Moreover, there is no committed
content inside here.

.gitignore

 *~
 *.bak
 .use-civicrm-setup
+/ext/
 backdrop/
 bower_components
 CRM/Case/xml/configuration