hw/xwin: Fix building with -fno-common

Provide an actual definition of noDriExtension where used, rather than a
tentative definition in a header, to fix compilation with -fno-common
(the default with gcc 10).
4 jobs for master in 6 minutes and 52 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #5782258

00:00:17

 
  Build And Test
passed autotools #5782259

00:06:35

passed meson #5782260

00:03:33

passed meson-noglamor #5782261

00:02:47