bufhelp: Suppress 'cast increases required alignment' warning

* cipher/bufhelp.h (buf_xor, buf_xor_2dst, buf_xor_n_copy): Cast
to larger element pointer through (void *) to suppress -Wcast-error.

Patch disables bogus warnings caused by -Wcast-error. We know that byte
pointers are properly aligned at these phases, or that hardware can handle
unaligned accesses.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>