With DTLSv1_get_timeout de-ctrl-ified, the type checker complains about
OPENSSL_timeval. Existing callers all use the real timeval.

Now that OPENSSL_timeval is not included in any public structs, simply
forward-declare timeval itself in ssl.h and pull in winsock2.h in internal
headers.

Change-Id: Ieaf110e141578488048c28cdadb14881301a2ce1
Reviewed-on: https://boringssl-review.googlesource.com/4682
Reviewed-by: Adam Langley <agl@google.com>