implicit casts from void*

I received on off-list report that commit e2809e3a101 causes an error
when building an extension written in C++, since $subject is in a
header file. The fix is simply to add an explicit cast, so I plan to
push the attached soon.

Bikeshedding: We could additionally change the pg_crc*.c files to make
them consistent, but I have not done that yet. It seems we prefer
explicit casts anyway but don't enforce that.

--
John Naylor
Amazon Web Services