Correction of setting up import of tax attributes from gnucash xea file

Summary:
KMyMoney expects the flag "Tax" to have an attribute value="Yes".

BUG:380286
FIXED-IN:4.8.3, 5.0.3

Test Plan: compiled and tested on linux

Reviewers: tbaumgart, wojnilowicz

Differential Revision: https://phabricator.kde.org/D16728