Make our perfect hash functions be valid C++.
While C is happy to cast "const void *" to "const unsigned char *"
silently, C++ insists on an explicit cast. Since we put these
functions into header files, cpluspluscheck whines about that.
Add the cast to pacify it.
Discussion: https://postgr.es/m/
b517ec3918d645eb950505eac8dd434e@gaz-is.ru