scripts/documentation-file-ref-check: ignore output dir

When there's no Documentation/output directory, the script will
complain about those missing references:

	Documentation/doc-guide/sphinx.rst: Documentation/output
	Documentation/doc-guide/sphinx.rst: Documentation/output
	Documentation/process/howto.rst: Documentation/output
	Documentation/translations/it_IT/doc-guide/sphinx.rst: Documentation/output
	Documentation/translations/it_IT/doc-guide/sphinx.rst: Documentation/output
	Documentation/translations/it_IT/process/howto.rst: Documentation/output
	Documentation/translations/ja_JP/howto.rst: Documentation/output
	Documentation/translations/ko_KR/howto.rst: Documentation/output

Those are false positives, so add an ignore rule for them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>