xquartz: Add stub ddxInputThread()

Omitted from 4ad21c32

hw/xquartz/darwin.c

@@ -826,3 +826,12 @@ ddxGiveUp(enum ExitCode error)
 {
     LogClose(error);
 }
+
+#if INPUTTHREAD
+/** This function is called in Xserver/os/inputthread.c when starting
+    the input thread. */
+void
+ddxInputThreadInit(void)
+{
+}
+#endif