Use X509_NAME, not struct X509_name_st.

Also include openssl/x509.h explicitly since we're using functions and types
from it.

BUG=none
R=henrike@webrtc.org, wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12859004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6569 4adac7df-926f-26a2-2b94-8c16560cd09d