AIX: Add flock to roken

AIX: Add flock to roken

Add the rk_flock() emulation function to roken, primarily for use
on the AIX build, but it will also be used for WIN32

Change-Id: Ia8f11e19bbe9fdf721a7b72660d860624139d9b1
Reviewed-on: http://gerrit.openafs.org/4368
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>