staging: comedi: make comedi_internal.h self-reliant

Add `#include`s and declare incomplete types to "comedi_internal.h" so
that .c files still compile when it is the first file included.

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>