Add xf86OSInputThreadInit to stub os-support as well

stub os support also needs to provide xf86OSInputThreadInit, omitted in
ea1527a8

hw/xfree86/os-support/stub/stub_init.c

@@ -24,3 +24,9 @@ void
 xf86UseMsg(void)
 {
 }
+
+void
+xf86OSInputThreadInit()
+{
+    return;
+}