iio: bmg160: IIO_BUFFER and IIO_TRIGGERED_BUFFER are required

This patch adds selects for IIO_BUFFER and IIO_TRIGGERED_BUFFER. Without
IIO_BUFFER, the driver does not compile.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>