wpa_gui-qt4: Include cstdio to avoid some compiler issues

It looks like some build systems do not find snprintf() here unless
cstdio is included explicitly.

wpa_supplicant/wpa_gui-qt4/peers.cpp

@@ -12,6 +12,7 @@
  * See README and COPYING for more details.
  */
 
+#include <cstdio>
 #include <QImageReader>
 
 #include "wpagui.h"