Fix master build
Fix master build

The master build of Couchbase was broken when libaio is installed:

    src/docio.cc: In member function 'int DocioHandle::_submitAsyncIORequests_Docio(docio_object*, size_t, async_io_handle*, int, size_t*, bool)':
    src/docio.cc:1473:23: error: 'aio_events' was not declared in this scope
             for (io_evt = aio_events; num_events > 0; --num_events, ++io_evt) {
                           ^

Change-Id: Ib9831464ecea9219aaaeb0361dd7fa94f5891b05
Reviewed-on: http://review.couchbase.org/64900
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

couchbase/forestdb,hisundar/forestdb,couchbase/forestdb,hisundar/forestdb,wurikiji/forestdb,wurikiji/forestdb