Use reference for iterating histogram Bins

* unique_ptr doesn't allow copying its pointer to outside. It causes
compilation failure of CV.

Change-Id: I4d5335d486cc82558a60a103ee93540685b23dd6
Reviewed-on: http://review.couchbase.org/75099
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Jung-Sang Ahn <jungsang.ahn@gmail.com>

src/filemgr.cc

@@ -4066,7 +4066,7 @@ void LatencyStats::getHistogram(FileMgr *file,
     }
     std::stringstream ss;
     ss << "{";
-    for (auto it : file->histStats[type]) {
+    for (auto& it : file->histStats[type]) {
         if (it->count()) {
             ss << "(" << it->start() << "µs - " << it->end() << "µs) : ";
             ss << it->count() << "; ";