table/cuckoo_table_reader.cc: pass func parameter by ref

Fix for:

[table/cuckoo_table_reader.cc:198]: (performance) Function
 parameter 'file_data' should be passed by reference.

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

table/cuckoo_table_reader.cc

@@ -191,7 +191,7 @@ class CuckooTableIterator : public Iterator {
 
  private:
   struct BucketComparator {
-    BucketComparator(const Slice file_data, const Comparator* ucomp,
+    BucketComparator(const Slice& file_data, const Comparator* ucomp,
                      uint32_t bucket_len, uint32_t user_key_len,
                      const Slice target = Slice())
       : file_data_(file_data),