Add missing 3.15.5 to 3.15.6 upgrade script (placeholder, no changes)

Signed-off-by: Chris Sharp <[email protected]>

Author: chrissharp123

Open-ILS/src/sql/Pg/version-upgrade/3.15.5-3.15.6-upgrade-db.sql

@@ -0,0 +1,7 @@
+--Upgrade Script for 3.15.5 to 3.15.6
+\set eg_version '''3.15.6'''
+BEGIN;
+
+INSERT INTO config.upgrade_log (version, applied_to) VALUES ('3.15.6', :eg_version);
+
+COMMIT;