Libev: Also check for include/libev

Change-Id: Ie1b2d9a1d902b6033f3cd977a2e0c07c72d0c033
Reviewed-on: http://review.couchbase.org/33391
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>