iio/imu: inv_mpu6050 depends on IIO_BUFFER

Fix:

drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c: In function ‘inv_mpu6050_read_fifo’:
drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c:176:3: error: implicit declaration of
function ‘iio_push_to_buffers’ [-Werror=implicit-function-declaration]

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>