substitute fixed version numbers in qdocconf files with variables

Change-Id: I2424061251bc55cc2196c483ba421bef831a36b6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/xmlpatterns/doc/qtxmlpatterns.qdocconf

@@ -32,7 +32,7 @@ qhp.qtxmlpatterns.file = qtxmlpatterns.qhp
 
 # Namespace for the output file. This namespace is used to distinguish between
 # different documentation files in Creator/Assistant.
-qhp.qtxmlpatterns.namespace = org.qt-project.qtxmlpatterns.501
+qhp.qtxmlpatterns.namespace = org.qt-project.qtxmlpatterns.$QT_VERSION_TAG
 
 # Title for the package, will be the main title for the package in
 # Assistant/Creator.