Bug 16007: Adding back column 'letternumber' to table 'overduerules_transport_types'

 Bug 16007: Adding back column 'letternumber' to table 'overduerules_transport_types'

Bug 13624 definitely broke the master by removing the column `overduerules_transport_type`.`letternumber` from kohastructure.sql.
This patch aims to fix the problem by adding the column back on systems which had their 'letternumber' removed.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>