zc_hashtable.c: add missing zc_error() argument in zc_hashtable_remove()

It still work because zc_error() will print something on stack if required
argument didn't given, but it does not do things we expected.
Merge pull request #41 from mofaph/add-missing-argument

zc_hashtable.c: add missing zc_error() argument in zc_hashtable_remove()