temp/iio-sensor-proxy: Support set_polling (MR 5853)

In order to reduce the power usage when using sensors let's implement
set_polling as described [1]. The patch was linked in the MR that adds
libssc support [2] and is from [3].

An alternative implementation is also avaliable [4], but that seems to
have GLib threading issues [5].

1: #2455 (comment 453587)
2: https://gitlab.freedesktop.org/hadess/iio-sensor-proxy/-/merge_requests/381#note_2567822
3: https://gitlab.freedesktop.org/verdre/iio-sensor-proxy/-/commit/7a7a5f3ea04ff4c134f3dbbbc005b8c26d0af300
4: https://gitlab.freedesktop.org/verdre/iio-sensor-proxy/-/commit/74a5860474b6e79cd6e3be13d1ff8dd9cd810b8b
5: https://mastodon.social/@verdre/113390737074625004



Signed-off-by: Alistair Francis <alistair@alistair23.me>
[ci:skip-build]: already built successfully in CI