fix gcc build

Index: contrib/backward-cpp/backward.hpp
--- contrib/backward-cpp/backward.hpp.orig
+++ contrib/backward-cpp/backward.hpp
@@ -79,6 +79,7 @@
 #include <algorithm>
 #include <cctype>
 #include <cstdio>
+#include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <fstream>
