krb5: use new krb5_time_abs() function

From Apple Heimdal-597.121.1: replace calls to labs() with krb5_time_abs()

lib/krb5/rd_cred.c

@@ -222,7 +222,7 @@ krb5_rd_cred(krb5_context context,
 
 	if (enc_krb_cred_part.timestamp == NULL ||
 	    enc_krb_cred_part.usec      == NULL ||
-	    labs(*enc_krb_cred_part.timestamp - sec)
+	    krb5_time_abs(*enc_krb_cred_part.timestamp, sec)
 	    > context->max_skew) {
 	    krb5_clear_error_message (context);
 	    ret = KRB5KRB_AP_ERR_SKEW;