cmake: don't add event2 to include path. Add parent

Change-Id: I9ae4d59bb5830177c81e466574a757f2433c598d
Reviewed-on: http://review.couchbase.org/33296
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>