x11-servers/xwayland-devel: update to 21.0.99.1.923

Explicit sync requires https://github.com/freebsd/drm-kmod/issues/278
and currently disabled at build time due to missing <linux/dma-buf.h>

Changes: https://gitlab.freedesktop.org/xorg/xserver/-/compare/052909ab9...c7d56b0e2
(cherry picked from commit 7982b8922d80be5c96acc6151aa3301040070dae)
(cherry picked from commit 4eca3319032d9570e886222cb61baafbed1226cd)