tests/gpgscm: Fix use of pointer.

* tests/gpgscm/scheme-private.h (struct scheme): Use (void *) for
alloc_seg.
* tests/gpgscm/scheme.c (alloc_cellseg): Use (void *) for cp.  Use
(void *) for coercion of address calculation.

In old C language, (char *) means an address. In modern C, it's
specifically an address with alignment=1. It's good to use (void *) for
an address, because newer compiler emits warnings. Note: in this
particular case, it is just a warning and the code is safe against
invalid alignment, though.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>