Update deskutils/latte-dock to latest upstream release

There was a 0.9.6 which showed up briefly, skip to 0.9.7 released
yesterday. Release notes are at

https://psifidotos.blogspot.com/2019/12/latte-bug-fix-release-v097.html

There's a handful of bug fixes and prep-work for Qt 5.14 (which isn't
on our radar for KDE-FreeBSD yet).