.gitignore: ignore docs/built created for in-tree builds

Commit 1290e671 creates 'docs/built' for in-tree builds of
Sphinx manuals but did not ignore it.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190314104622.101715-2-sgarzare@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>