Staging: comedi: fix build error in comedilib.h

If anyone included this file, it would break the build.
This patch fixes the issue.

Cc: David Schleef <ds@schleef.org>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>