Add xf86OSInputThreadInit to stub os-support as well

stub os support also needs to provide xf86OSInputThreadInit, omitted in
ea1527a8
4 jobs for threadinit-stubos in 6 minutes and 56 seconds (queued for 6 minutes and 48 seconds)
Status Job ID Name Coverage
  Docker Image
passed #169973
debian-testing

00:00:32

 
  Build And Test
passed #169974
autotools-build-and-test

00:04:52

passed #169975
meson-build-and-test

00:01:31

 
  External
passed continuous-integration/appveyor/branch