ldb: only used a->syntax->index_format_fn if GUID indexing is enabled

ldb: only used a->syntax->index_format_fn if GUID indexing is enabled

Only enable ORDERED_INTEGER and index_format_fn functionality if GUID
indexing is enabled.  This is in line with the rest of ldb_kv which binds
the new ORDERED_INTEGER to GUID indexed databases, and allows a practical
way to create the old index format (by disabling the GUID index).

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>