Linux 3.10: Include linux/aio.h directly

This file needs bits from linux/aio.h, but gets them indirectly
from the inclusion of aio.h by sched.h.  That gets removed for
Linux 3.10.

Since this header is not of general interest elsewhere, just include
it directly here.

Change-Id: I432bde282a1c5425ba6780bca28e308bf70a79e7
Reviewed-on: http://gerrit.openafs.org/9912
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>