[FIX] account_financial_report: add index to report_move_id

When running `DELETE FROM report_journal_ledger_move` this column is
checked repeatedly, even though the table is empty. On our setup that
caused the query to take hours. Adding an index solved it.

account_financial_report/report/journal_ledger.py

@@ -722,6 +722,7 @@ class ReportJournalLedgerMoveLine(models.TransientModel):
         comodel_name='report_journal_ledger_move',
         required=True,
         ondelete='cascade',
+        index=True,
     )
     move_line_id = fields.Many2one(
         comodel_name='account.move.line',