Don't forward declare RWLockWrapper in clock.h

Include rw_lock_wrapper.h instead of forward declaring. This is to
come around problems with thread annotations in some build systems.

BUG=3516
R=mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17809004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6545 4adac7df-926f-26a2-2b94-8c16560cd09d