examples: fix 'control reaches end of non-void function' warning in cairo example

tests/examples/cairo/cairo_overlay.c

@@ -167,4 +167,6 @@ main (int argc, char **argv)
 
   gst_element_set_state (pipeline, GST_STATE_NULL);
   gst_object_unref (pipeline);
+
+  return 0;
 }