krb5: add initscripts-functions to RDEPENDS

As this recipe doesn't inherit update-rc.d, we need to add to its
runtime dependency initscripts-functions ourselves. Otherwise, we
would spot errors in systemd systems when we execute commands like
`systemctl start krb5-kdc'.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>