Add prototypes for the atomic functions for win32

No implementation currently. Just trying to make the
other stuff compile so far.

Change-Id: I168c92e0f36f610a6c5def57d16b8985b319139b
Reviewed-on: http://review.couchbase.org/31196
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>