Explain why QSystemLocale needs to be an export. Unexport
QLocaleId::fromName() now that qttools no longer uses it.
QLocalePrivate was only Q_CORE_EXPORT for the sake of one use in
tst_QLocale, that can now be replaced by a call to a public QLocale
method.
Change-Id: I1efe8ee03488acfc6e1c36661f6e956a86e191c3
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>