klog.krb5: cast get_cred_keylen to unsigned

klog.krb5: cast get_cred_keylen to unsigned

get_cred_keylen can yield a type besides an unsigned int (such as a
size_t on heimdal). But we are printing it with %u, which causes a
warning, so cast it to an unsigned int.

Change-Id: I7b89de5b0b163b9532ac347e9c56e865cb58f266
Reviewed-on: http://gerrit.openafs.org/6410
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>