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>

drivers/staging/comedi/comedi_internal.h

 #ifndef _COMEDI_INTERNAL_H
 #define _COMEDI_INTERNAL_H
 
+#include <linux/compiler.h>
 #include <linux/types.h>
 
 /*
  * various internal comedi stuff
  */
+
+struct comedi_buf_map;
+struct comedi_devconfig;
+struct comedi_device;
+struct comedi_insn;
+struct comedi_rangeinfo;
+struct comedi_subdevice;
+struct device;
+
 int do_rangeinfo_ioctl(struct comedi_device *dev,
 		       struct comedi_rangeinfo __user *arg);
 struct comedi_device *comedi_alloc_board_minor(struct device *hardware_device);