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>