doc-rst:c-domain: fix sphinx version incompatibility

The self.indexnode's tuple has changed in sphinx version 1.4, from a
former 4 element tuple to a 5 element tuple.

https://github.com/sphinx-doc/sphinx/commit/e6a5a3a92e938fcd75866b4227db9e0524d58f7c



Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>