Fix old-style definition warning for xf86OSInputThreadInit()

../hw/xfree86/os-support/stub/stub_init.c: In function ‘xf86OSInputThreadInit’:
../hw/xfree86/os-support/stub/stub_init.c:29:1: warning: old-style function definition [-Wold-style-definition]

(cherry picked from commit 7c266caf)
3 jobs for warning-fix-1.20 in 5 minutes and 4 seconds