s4-provision: Ensure the dummy main-domain DB used for DLZ has an @INDEXLIST

s4-provision: Ensure the dummy main-domain DB used for DLZ has an @INDEXLIST

The other databases are created from copies of the main provision, but this one
is not, so did not previously get a valid @INDEXLIST.

This is important as otherwise we will not correctly notice support for
the GUID index or new DSDB features in @SAMBA_DSDB as this is gated
on seeing @SAMBA_FEATURES_SUPPORTED in @INDEXLIST.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>