Tests: Add ox_announce_public_key stub

tests/unittests/xmpp/stub_ox.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+#include <glib.h>
+#include <stdlib.h>
+
+gboolean 
+ox_announce_public_key(const char* const filename)
+{
+    return FALSE;
+}