hw/xfree86: Fix the building of int10 with -fno-common

Provide an actual definition of IOPortBase, rather than a tentative
definition in a header, to fix compilation with -fno-common
(the default with gcc 10).
4 jobs for mips-ftbfs in 11 minutes and 32 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed #7750661
debian-buster

00:05:17

 
  Build And Test
passed #7750662
autotools

00:06:15

passed #7750663
meson

00:03:34

passed #7750664
meson-noglamor

00:02:45