Add the CTX parameter back to EC_GROUP_cmp.

It was a mistake to remove this in the first place.

Change-Id: Icd97b4db01e49151daa41dd892f9da573ddc2842
Reviewed-on: https://boringssl-review.googlesource.com/3541
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>