Log message:
Add graphics/libimagequant-c v2.18.0

Follow the example of graphics/librsvg{,-c} and re-add the last C
version of imagequant-turned-rust, for the sake of platforms which
cannot reasonably build or run rusted tools for lack of support or
resources.

See also the
<https://mail-index.netbsd.org/pkgsrc-users/2023/06/02/msg037421.html>
thread.