Remove implicit cast from 'void *'

Remove implicit cast from 'void *'

Commit e2809e3a101 added code to a header which assigns a pointer
to void to a pointer to unsigned char. This causes build errors for
extensions written in C++. Fix by adding an explicit cast.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CANWCAZaCq9AHBuhs%3DMx7Gg_0Af9oRU7iAqr0itJCtfmsWwVmnQ%40mail.gmail.com
Backpatch-through: 18