x11/swaylock-plugin: update to 1.8.1

--grace flag isn't supported on FreeBSD due to missing (e)logind but
can probably be ported to basu + consolekit2 later.

Changes: https://github.com/mstoeckl/swaylock-plugin/releases/tag/v1.8.1
Reported by: GitHub (watch releases)

(cherry picked from commit 4973746ecf8edfc3dd3d20e4d67b03e0119d9b8f)