unbreak with gcc 4.9

Index: libtest/test.hpp
--- libtest/test.hpp.orig
+++ libtest/test.hpp
@@ -59,6 +59,7 @@
 
 #include <cstdio>
 #include <cstdlib>
+#include <cstdarg>
 #include <arpa/inet.h>
 
 #include <libtest/visibility.h>