Work around VS2017 warning for unused reference

Summary:
For #3407
Closes https://github.com/facebook/rocksdb/pull/3425

Differential Revision: D6836900

Pulled By: gfosco

fbshipit-source-id: 7bcaf7a1beeeeabb7c05584f2745e7b4a2473497

db/column_family.cc

@@ -1060,11 +1060,13 @@ ColumnFamilySet::~ColumnFamilySet() {
   while (column_family_data_.size() > 0) {
     // cfd destructor will delete itself from column_family_data_
     auto cfd = column_family_data_.begin()->second;
-    bool last_ref __attribute__((__unused__)) = cfd->Unref();
+    bool last_ref __attribute__((__unused__));
+    last_ref = cfd->Unref();
     assert(last_ref);
     delete cfd;
   }
-  bool dummy_last_ref __attribute__((__unused__)) = dummy_cfd_->Unref();
+  bool dummy_last_ref __attribute__((__unused__));
+  dummy_last_ref = dummy_cfd_->Unref();
   assert(dummy_last_ref);
   delete dummy_cfd_;
 }