Bug 22634: Standardize table creation for stockrotation* tables in kohastructure.sql

Bug 22634: Standardize table creation for stockrotation* tables in kohastructure.sql

The existing pattern is:
DROP TABLE IF EXISTS ...
CRATE TABLE

Stockrotation was using:
CREATE TABLE IF NOT EXISTS ...
which would not recreate the table in a database it's already
existing in possibly causing mismatches in table structure.

To test:
- Drop your database
- Run through the web installer
- Make sure there are no Database/SQL errors
- Makse sure the 4 stockrotation tables have been created

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>