Linux v4.11: cred.h is no longer included in sched.h

Linux v4.11: cred.h is no longer included in sched.h

With Linux commit e26512fea5bcd6602dbf02a551ed073cd4529449, cred.h is no
longer included in sched.h.

Several components of libafs which require cred.h were picking it by
including sched.h.

Instead, explicitly add an include for cred.h. cred.h begins with a
customary one-shot to prevent multiple loads:

 #ifndef _LINUX_CRED_H
 #define _LINUX_CRED_H

Therefore we don't need a new autoconf test or preprocessor conditional
to prevent redundant includes on older Linux releases.

Change-Id: Ifc496c83141d2cfbd417133feb6d87c1146e5014
Reviewed-on: https://gerrit.openafs.org/12574
Tested-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Joe Gorse <jhgorse@gmail.com>
Tested-by: Joe Gorse <jhgorse@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>