LINUX: Include key-related headers in osi_compat.h

LINUX: Include key-related headers in osi_compat.h

Include keyring-related headers in osi_compat.h, so we get the proper
types defined for keyring-related functions. Also only define
keyring-related functions if we have keyring support.

Reviewed-on: http://gerrit.openafs.org/3895
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 358f2a23079f940e9adb741d2526895d620d1ced)

Change-Id: Ice7b37feb23acef73d64d923243ee622db657b5d
Reviewed-on: http://gerrit.openafs.org/4070
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>