x11-servers/xlibre-server: fix build on armv7

This ensures USE_DEV_IO=1 is set on armv7.

See also: b632464b29fc330113280495ef76d0f825058ae8
Approved by: portmgr (build fix blanket)
MFH: 2026Q1

(cherry picked from commit e437c6ec629f7d5b9a0617b15e5da69fa2348fad)