16f774f8bf adds forward declarations for
everything in x509.h, but the typedefs are still in x509.h. Some versions of
clang flag the duplicate typedefs in C code.
Change-Id: Ib6684a238681d8c4fb1f0f91c3a6110013b3f4d6
Reviewed-on: https://boringssl-review.googlesource.com/5580
Reviewed-by: Adam Langley <agl@google.com>