Silence warning about unnecessary use of the containing namespace

ICC complained:
src/plugins/qmldesigner/designercore/include/propertycontainer.h(52): warning #1098: the qualifier on this friend declaration is ignored
      friend CORESHARED_EXPORT QDataStream &QmlDesigner::operator<<(QDataStream &stream, const PropertyContainer &propertyContainer);
                                            ^

The solution is to simply remove the namespace qualifier when the
class is already inside that namespace.

Change-Id: I2256ebe072b3f8db5288f72acb1388eabcb8325c
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>