wlroots update - new repo

[florhizome]

Hi Guix,
guix' wlroots is pretty old (0.14.1 compared to the newest version 0.15.1). I 
have several packages (tools,
compositors) that require a newer version or at least profit from it.
Maybe it's relevant that they switched repo to
https://gitlab.freedesktop.org/wlroots/wlroots/ , and so "guix refresh"
doesn't find the new versions ? ;)
I would also want to encourage opening new modules for wayland
compositors and/or tooling. I just think it's more clean then appending
them to "gnu/packages/wm.scm"

Kind regards,
florhizome