Fix undesirable copy with auto in timestamp extractor

Change-Id: I7ec3dcd977dca5870c9d4f7e4dc235f8bbbf20dc
diff --git a/aos/events/logging/timestamp_extractor.cc b/aos/events/logging/timestamp_extractor.cc
index 2b84975..232b842 100644
--- a/aos/events/logging/timestamp_extractor.cc
+++ b/aos/events/logging/timestamp_extractor.cc
@@ -18,7 +18,7 @@
 
 std::string LogFileVectorToString(std::vector<logger::LogFile> log_files) {
   std::stringstream ss;
-  for (const auto f : log_files) {
+  for (const auto &f : log_files) {
     ss << f << "\n";
   }
   return ss.str();