Fix the type of ASN1_i2d_bio's last argument.

It's supposed to be void*. The only reason this was working was that it was
only called in C which happily casts from void* to T*. (But if called in C++ in
a macro, it breaks.)

Change-Id: I7f765c3572b9b4815ae58da852be1e742de1bd96
Reviewed-on: https://boringssl-review.googlesource.com/5760
Reviewed-by: Adam Langley <agl@google.com>