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>