xquartz: Add stub ddxInputThread()

Omitted from 4ad21c32
4 jobs for xquartz-ddxinputthreadinit in 6 minutes and 4 seconds (queued for 1 second)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #266605

00:00:36

 
  Build And Test
passed autotools-build-and-test #266606

00:05:27

passed meson-build-and-test #266607

00:01:27

 
  External
passed continuous-integration/appveyor/branch