blockchain_stats: don't use gmtime_r on Windows

In some cases, it doesn't like it (I don't know the details).

Factor into a new epee function

contrib/epee/include/misc_os_dependent.h

@@ -122,6 +122,15 @@ namespace misc_utils
 		return boost::lexical_cast<std::string>(GetCurrentThreadId());
 #elif defined(__GNUC__)  
 		return boost::lexical_cast<std::string>(pthread_self());
+#endif
+	}
+
+	inline bool get_gmt_time(time_t t, struct tm &tm)
+	{
+#ifdef _WIN32
+		return gmtime_s(&tm, &t);
+#else
+		return gmtime_r(&t, &tm);
 #endif
 	}
 }