Don't manually extern OPENSSL_ia32cap_P.

This probably snuck in when adapting the code from upstream. There's a header
file for it now. (Also it's uint32_t now rather than unsigned int.)

Change-Id: Ie8f45bc7a88988744174182a70512c0eff37cc1c
Reviewed-on: https://boringssl-review.googlesource.com/2441
Reviewed-by: Adam Langley <agl@google.com>