rxgen: skip ubik for KERNEL

Declare ubik rxgen stubs only for !KERNEL, UKERNEL doesn't need them anymore.
Don't declare ubik_client or #include ubik.h on KERNEL or UKERNEL.

Change-Id: I0b1587eb46e9efbf627f04c74e0d76f9858bffd0
Reviewed-on: http://gerrit.openafs.org/9954
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>