make use of qtHaveModule()

Change-Id: I14da5455b7dd4681254e1652395df5f5c65746bd
Reviewed-by: Tasuku Suzuki <stasuku@gmail.com>
Reviewed-by: Jonas Rabbe <jonas.rabbe@gmail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

src/plugins/wmf/wmf.pro

@@ -1,6 +1,6 @@
 TARGET = wmfengine
 QT += multimedia-private network
-!isEmpty(QT.widgets.name) {
+qtHaveModule(widgets) {
     QT += multimediawidgets-private
     DEFINES += HAVE_WIDGETS
 }