Set the default QPA platform to minimal for the qcollectiongenerator

Since the minimal platform is used for the qcollectiongenerator this
should be set in the pro file to ensure it will still work when this is
statically built.

Task-number: QTBUG-60926
Change-Id: I75481fe2600aecd573a429b9b9d6ec6acb803e6c
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/assistant/qcollectiongenerator/qcollectiongenerator.pro

@@ -5,5 +5,6 @@ SOURCES += ../shared/helpgenerator.cpp \
     ../shared/collectionconfiguration.cpp
 HEADERS += ../shared/helpgenerator.h \
     ../shared/collectionconfiguration.h
+QTPLUGIN.platforms = qminimal
 
 load(qt_tool)