Add QT_DEFAULT_QPA_PLUGIN to generated qconfig.pri

Configure will now generate QT_DEFAULT_QPA_PLUGIN qmake variable
to specify the default QPA plugin.

"CONFIG += qpa_default_plugin" statement in application .pro file
will add the default QPA plugin into QTPLUGINS.

"CONFIG += qpa_minimal_plugin" statement in application .pro file
will add the minimal QPA plugin into QTPLUGINS.

Task-number: QTBUG-28131
Change-Id: I12a241005f30b37467d783b50f0369b47e605e68
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>