Bug 21396: Add 2 missing use statements in Koha::Account

Bug 21396: Add 2 missing use statements in Koha::Account

1. require C4::Letters must be use C4::Letters (from bug 19191)

2. There is call to C4::Circulation::ReturnLostItem, but no use of
C4::Circulation (from bug 20631)

Test plan:
finger crossed, I ran the tests from t/db_dependent/Koha and they all
passed

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>