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.
Change-Id: Ia7e38160492b1e03cdb257e4b2bef4d18c4a28fb
Reviewed-on: https://gerrit.openafs.org/12593
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>