QmlJS: Only try qmldump on Desktop and Simulator Qt.

In particular, don't complain about a missing qmldump binary on other
platforms.

Change-Id: Ie2e96bcb67e609aa5aed31510b06139d2d9ce77e
Reviewed-on: http://codereview.qt-project.org/5327


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>