Linux: only include cred.h if it exists

Linux: only include cred.h if it exists

Commit c89fd17df1032ec2eacc0d0c9b73e19c5e8db7d2 introduced an explicit
include of linux/cred.h since the latest kernel no longer includes it
implicitly in sched.h. Alas, older kernels (like 2.6.18) don't have this
file. Add a configure test for the existence of cred.h and only include
it if actually present.

Reviewed-on: https://gerrit.openafs.org/12593
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 6b7b4239ab22fbb301e3b50e2ca4072445ba4e9e)

Change-Id: I64970ba471180d32fa5af5445e7604bbe8511b32
Reviewed-on: https://gerrit.openafs.org/12598
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>