[media] conf_nitpick.py: ignore external functions used on mediactl

There are some functions/macros used by the mediactl documentation
that are alien to the media subsystem. Ignore them.

After this patch, the media core will only complain about this
static function:
	Documentation/media/kapi/mc-core.rst:97: WARNING: c:func reference target not found: media_devnode_release

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>