staging:iio:iio_core.h make less dependent on other included headers

There are a lot of pointers to structures used in here that are not
declared unless a particular header is included first.  Deal with
the IIO specific ones by putting in forward declarations and the
other ones by including kernel.h and device.h.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/iio/iio_core.h

@@ -12,6 +12,12 @@
 
 #ifndef _IIO_CORE_H_
 #define _IIO_CORE_H_
+#include <linux/kernel.h>
+#include <linux/device.h>
+
+struct iio_chan_spec;
+struct iio_dev;
+
 
 int __iio_add_chan_devattr(const char *postfix,
 			   struct iio_chan_spec const *chan,