x11-servers/xorg-server: fix build on armv6/armv7
This reinstates USE_DEV_IO=1 on arm, a patch that was dropped in the
switch from autotools to meson.
Approved by: portmgr (build fix blanket)
MFH: 2023Q4
(cherry picked from commit b632464b29fc330113280495ef76d0f825058ae8)