dmx: Fix some redeclaration warnings from gcc 11

Of the form:

    ../hw/dmx/config/xdmxconfig.c:68:26: warning: redundant redeclaration of ‘dmxConfigEntry’ [-Wredundant-decls]
       68 | extern DMXConfigEntryPtr dmxConfigEntry;
          |                          ^~~~~~~~~~~~~~
5 jobs for warning-fixes in 7 minutes and 59 seconds (queued for 11 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #12863105

00:00:37

 
  Build And Test
passed autotools #12863106

00:07:21

passed meson #12863107

00:04:08

passed meson-noglamor #12863108

00:02:13

passed mingw-cross-build #12863109

00:01:02