Do not use system libxml if not ICU capable

Using a libxml without ICU capability will break some websites, so
better fall back to bundled libxml in that case.

Task-number: QTBUG-59094
Change-Id: Ia24c4045ed9eb9731367cca07d8467ea9b785f2e
Reviewed-by: Florian Bruhin <qt-project.org@the-compiler.org>
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>