w32/strptime: Fix for 64 bit pointers

w32/strptime: Fix for 64 bit pointers

The __UNCONST() stolen from NetBSD assumes unsigned long can hold a
pointer.  Not true with Win64's LLP64 data model.  There, we cast the
64 bit pointer to 32 bits and back.  Works only because Windows puts
the stack at a very low address, and the casts don't actually change
the pointer.

Dumb it down to a straight cast to void * for safety.

Thanks to Harald Katzer and Ron Koenderink for their help figuring out
the bug's impact.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>