Remove redundant declaration of OPENSSL_ia32cap_P from e_aes.c.

Some compilers warn about this duplicate declaration in some
configurations.

Change-Id: I3ff038a5feb0b2632b140e6632ea48acb0dcc118
Reviewed-on: https://boringssl-review.googlesource.com/5551
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>