move i3 sync code into sync_respond (for following commits)

include/sync.h

@@ -0,0 +1,14 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * sync.c: i3 sync protocol: https://i3wm.org/docs/testsuite.html#i3_sync
+ *
+ */
+#pragma once
+
+#include <xcb/xcb.h>
+
+void sync_respond(xcb_window_t window, uint32_t rnd);