Don't use struct names in ssl.h.

The type names are perfectly serviceable. Most of them are
forward-declared in base.h.

Change-Id: Id03f5039a2d1bab82c68ade074a0e26cd3ab5ad9
Reviewed-on: https://boringssl-review.googlesource.com/4783
Reviewed-by: Adam Langley <agl@google.com>