This could lead to problem when simplehash.h is used to define two
different types of hashtable visible in the same translation unit.

Reported-By: Josh Soref
Discussion: https://postgr.es/m/CACZqfqCC7WdBAY=rQePb9-qW1rjdaTdHsV5KoVejHkDb6qrtOg@mail.gmail.com