db_test.cc: pass parameter by reference

Fix for:

[db/db_test.cc:6141]: (performance) Function parameter
 'key' should be passed by reference.

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

db/db_test.cc

@@ -6138,7 +6138,7 @@ class WrappedBloom : public FilterPolicy {
   const FilterPolicy* filter_;
   mutable uint32_t counter_;
 
-  rocksdb::Slice convertKey(const rocksdb::Slice key) const {
+  rocksdb::Slice convertKey(const rocksdb::Slice& key) const {
     return key;
   }
 };