Fix #863

Author: yhirose

httplib.h

@@ -3775,7 +3775,7 @@ inline std::string random_string(size_t length) {
                            "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                            "abcdefghijklmnopqrstuvwxyz";
     const size_t max_index = (sizeof(charset) - 1);
-    return charset[static_cast<size_t>(rand()) % max_index];
+    return charset[static_cast<size_t>(std::rand()) % max_index];
   };
   std::string str(length, 0);
   std::generate_n(str.begin(), length, randchar);