samba4: add mandatory option per CVE-2018-16853

samba4: add mandatory option per CVE-2018-16853

Since 4.9.3, Samba AD-DC with MIT Kerberos will refuse to build unless
--with-experimental-mit-ad-dc is provided to the configure command.

The mandatory requirement was introduced in response to a report that
a user in a Samba AD domain can crash the KDC when Samba is built in
the non-default MIT Kerberos configuration:
https://www.samba.org/samba/security/CVE-2018-16853.html

This requirement was introduced in Samba commit
c5370a4349d381ba3b64b063dc28a2c54cfacdfc.

Signed-off-by: Val Kulkov <val.kulkov@gmail.com>