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>