Add missing depends on currency to the TaxableMixin._round_taxes method

Closes #14348
(grafted from c3e0f5727931a8674b377930ff9df896f95cbdbb)

--HG--
branch : 7.0

modules/account/tax.py

@@ -1253,6 +1253,7 @@ class TaxableMixin(object):
 
         return _TaxKey(**line)
 
+    @fields.depends('currency')
     def _round_taxes(self, taxes):
         if not self.currency:
             return