kauth: Resolve date signedness warning in SetFields

kauth: Resolve date signedness warning in SetFields

Resolves this warning:

admin_tools.c: In function ‘SetFields’:
admin_tools.c:611:30: warning: pointer targets in passing argument 2 of ‘ktime_DateToInt32’ differ in signedness [-Wpointer-sign]
  code = ktime_DateToInt32(s, &expiration);
                              ^
In file included from /home/anders/wd/openafs/include/afs/afsutil.h:84:0,
                 from admin_tools.c:39:
/home/anders/wd/openafs/include/afs/afsutil_prototypes.h:101:18: note: expected ‘afs_int32 *’ but argument is of type ‘afs_uint32 *’
 extern afs_int32 ktime_DateToInt32(char *adate, afs_int32 * aint32);
                  ^

Change-Id: Id24e7a6cd1ab2291c0c05d3835f4ad7fddfec8d7
Reviewed-on: http://gerrit.openafs.org/11956
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>