qhelpgenerator: Fix warning about qpa_minimal_plugin usage

  Project WARNING: CONFIG+=qpa_minimal_plugin is deprecated. Use QTPLUGIN.platforms=qminimal instead.

Change-Id: I9f07c5033e0022e00c45e52570fe001856f13ca8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/assistant/qhelpgenerator/qhelpgenerator.pro

 QT += network help-private
 
-CONFIG += qpa_minimal_plugin
+QTPLUGIN.platforms = qminimal
 
 SOURCES += ../shared/helpgenerator.cpp \
            main.cpp