Staging: comedi: comedi_rt_task_context_t fix

Fix redefinition of macro comedi_rt_task_context_t

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