Fix copy with auto in for loop

Some compilers freak out about this.

Change-Id: I03f9ebbb2e6ff93ef3702288057978d00606d38f
diff --git a/aos/events/logging/logger_test.cc b/aos/events/logging/logger_test.cc
index f60a62a..5483969 100644
--- a/aos/events/logging/logger_test.cc
+++ b/aos/events/logging/logger_test.cc
@@ -452,17 +452,17 @@
     event_loop_factory_.SetTimeConverter(&time_converter_);
 
     // Go through and remove the logfiles if they already exist.
-    for (const auto file : logfiles_) {
+    for (const auto &file : logfiles_) {
       unlink(file.c_str());
       unlink((file + ".xz").c_str());
     }
 
-    for (const auto file :
+    for (const auto &file :
          MakeLogFiles(tmp_dir_ + "/relogged1", tmp_dir_ + "/relogged2")) {
       unlink(file.c_str());
     }
 
-    for (const auto file : pi1_reboot_logfiles_) {
+    for (const auto &file : pi1_reboot_logfiles_) {
       unlink(file.c_str());
     }