Fix DARWIN build with clang

In 1d8937b86050 we added a function call to kauth_cred_unref in the
DARWIN100 case (replacing a macro), but added the inclusion of
sys/kauth.h only when using versions older than DARWIN80.
On DARWIN100 and above, clang detects that the now-implicit function
declaration is in conflict with the actual prototype, which is included
later through afs/sysincludes.h when compiling the kernel rx code.

Since including sys/kauth.h seems to have been harmless for old versions,
just include it always.

Change-Id: I4b65f5405a3de918091f8c25440801d0ce9f90b8
Reviewed-on: http://gerrit.openafs.org/9676
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>