block-sha1/sha1.c: silence compiler complaints by casting void * to char *

block-sha1/sha1.c: silence compiler complaints by casting void * to char *

Some compilers produce errors when arithmetic is attempted on pointers to
void.  We want computations done on byte addresses, so cast them to char *
to work them around.

Signed-off-by: Brandon Casey <redacted>
Signed-off-by: Junio C Hamano <redacted>