community/sxmo-wlroots: bugfix backport for 3.15

There was a significant bug in sxmo-wlroots in v3.15 (https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/3402). This updated package should remedy the problem by adding a patch. Backporting this to v3.15 should ensure everything is in order for the upcoming postmarketOS v21.12 release.

Eventually, once our fixes are upstreamed into sway and a new sway gets released, this temporary fork will be obsolete entirely.