Fix event test for win32

Newer libevents defines EVENT__ and not _EVENT.
This is a temporary fix until we upgrade the dependency to
libevent2 for all platforms.

Change-Id: I868dfbec8c0f37c9f2580211d114a3f1b6c761a8
Reviewed-on: http://review.couchbase.org/34774
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>