Bug 29605: Add missing constraints from bug 15067

Bug 29605: Add missing constraints from bug 15067

This patch fixes a missed update to kohastructure.sql.

To test:
1. Checkout prior to bug 15067
(ab270c026be06f08745a14d1aea992877d20f812) would work.
2.  Run:
   $ reset_all
3. Checkout v21.11.00
4. Run:
   $ updatedatabase
   $ koha-mysql kohadev
   > SHOW CREATE TABLE language_subtag_registry;
=> SUCCESS: There's UNIQUE KEY uniq_lang (subtag, type)
5. Run:
   > SHOW CREATE TABLE language_descriptions;
=>  SUCCESS: There's UNIQUE KEY uniq_desc (subtag, type, lang)
6. Run:
   > SHOW CREATE TABLE language_rfc4646_to_iso639;
=> SUCCESS: There's UNIQUE KEY uniq_code (rfc4646_subtag, iso639_2_code)
7. Replicate a fresh install. Run:
   $ reset_all
8. Repeat 4, 5 and 6
=> FAIL: The UNIQUE constraints are not there!
9. Apply this patch
10. Run:
   $ reset_all
11. Repeat 8
=> SUCCESS: The UNIQUE constraints are there!
12. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>