staging: comedi: addi_watchdog.h: don't include "../comedidev.h"

staging: comedi: addi_watchdog.h: don't include "../comedidev.h"

The Comedi "addi_watchdog.h" header doesn't use anything form
"comedidev.h" apart from `struct comedi_subdevice`, which it only uses
to construct a corresponding pointer type within the parameter list of a
function prototype.  Just declare the structure type incompletely and
don't bother including the header file.

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>