Fix hashtree test missing function

The 1.4 branch riak_kv repo had an EQC test failing due to a missing function as a result of a backport. This fixes it.

You can verify by running only the hashtree test. Make sure you have quickcheck installed, then in riak_kv:

```
$ rebar deps_dir=.. skip_deps=true eunit suites=hashtree
or, if standalone riak_kv:
$ rebar skip_deps=true eunit suites=hashtree
```

Before and after applying this fix.