Fix build error

Author: yhirose

test/fuzzing/server_fuzzer.cc

@@ -39,6 +39,8 @@ class FuzzedStream : public httplib::Stream {
 
   socket_t socket() const override { return 0; }
 
+  time_t duration() const override { return 0; };
+
 private:
   const uint8_t *data_;
   size_t size_;