Add #defines for ASN.1 NULL and GENERALIZEDTIME.

Change-Id: Ic82ab5de4e231cdf6230ee7262c3c7539404d4a6
Reviewed-on: https://boringssl-review.googlesource.com/5020
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>