iio: buffer: Move iio_buffer_alloc_sysfs and iio_buffer_free_sysfs

The next patch will introduce new dependencies in iio_buffer_alloc_sysfs()
to functions which are currently defined after iio_buffer_alloc_sysfs(). To
avoid forward declarations move both iio_buffer_alloc_sysfs() and
iio_buffer_free_sysfs() after those function.

This is split into two patches one moving the functions and one adding the
dependencies to make review of the actual changes easier.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>