[IMP] account_tax_balance: add index

This indexes improve the display time of
account_tax_balance by a factor of 10 on a database
with 1M move lines.

account_tax_balance/models/__init__.py

@@ -4,3 +4,4 @@
 
 from . import account_move
 from . import account_tax
+from . import account_move_line