staging: comedi: das08.h: make self-reliant

The Comedi "das08.h" header file is included by drivers for the
ComputerBoards/MeasurementComputing and Keithley Metrabyte boards in the
DAS08 series.  It does not compile cleanly when it is the first header
included by the ".c" file.  It uses `struct comedi_device *` in the
parameter list of a function prototype, so just declare `struct
comedi_device` as an incomplete type.  It also uses `bool`, so include
<linux/types.h> to declare it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/das08.h

@@ -19,6 +19,10 @@
 #ifndef _DAS08_H
 #define _DAS08_H
 
+#include <linux/types.h>
+
+struct comedi_device;
+
 /* different ways ai data is encoded in first two registers */
 enum das08_ai_encoding { das08_encode12, das08_encode16, das08_pcm_encode12 };
 /* types of ai range table used by different boards */