Add ERR_get_next_error_library back.

It was removed in the fork but it turned out to need it.

Change-Id: I21030c8d5befecb63f2c40a59963bec1da1d96fb
Reviewed-on: https://boringssl-review.googlesource.com/1081
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>