db_ttl_impl.h: pass func parameter by reference

Fix for:

[utilities/ttl/db_ttl_impl.h:209]: (performance) Function parameter
 'merge_op' should be passed by reference.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

utilities/ttl/db_ttl_impl.h

@@ -206,7 +206,7 @@ class TtlCompactionFilterFactory : public CompactionFilterFactory {
 class TtlMergeOperator : public MergeOperator {
 
  public:
-  explicit TtlMergeOperator(const std::shared_ptr<MergeOperator> merge_op,
+  explicit TtlMergeOperator(const std::shared_ptr<MergeOperator>& merge_op,
                             Env* env)
       : user_merge_op_(merge_op), env_(env) {
     assert(merge_op);