Add index to billing fees.

Author: Kirill Solomko <ksolomko@sipwise.com>

db_scripts/diff/13841.down

@@ -0,0 +1,2 @@
+USE billing;
+DROP INDEX showfeesc_idx;

db_scripts/diff/13841.up

@@ -0,0 +1,2 @@
+USE billing;
+CREATE INDEX showfeesc_idx ON billing_fees(billing_profile_id, billing_zone_id, destination);