make it possible to disable XML schema usage

Removing XML schema functionality reduces the size of
libQt5XmlPatterns.so considerably:

Linux desktop release mode:

with XML schema:    4.3 MB
without XML schema: 3.2 MB

BlackBerry10 release mode:

with XML schema:    2.6 MB
without XML schema: 1.9 MB

XML schema might not be used, especially on a mobile / embedded
device, as opposed to the rest of xmlpatterns (e.g. XPath through
QML XmlListModel).

Task-number: QTBUG-28068
Change-Id: I5024a822179e7241e592f079efe3adef8f661c70
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>