heimdal: Use krb5_free_default_realm() for free()

heimdal: Use krb5_free_default_realm() for free()

The resource allocated by krb5_default_default_realm() should be
free()'d by krb5_free_default_realm() instead of plain free()
for better readability.

Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>