Fix compilation with Qt 6.8 (dev)

We need to export the SSLLabel with Qt 6.8 otherwise we can't use findChild.

Fix:

 40 In file included from /home/carl/kde6/usr/include/QtCore/QObject:1,                                                                                                                                                                    
 41                  from /home/carl/kde6/src/libkdepim/src/progresswidget/autotests/statusbarprogresswidgettest.h:9,                                                                                                                      
 42                  from /home/carl/kde6/src/libkdepim/src/progresswidget/autotests/statusbarprogresswidgettest.cpp:7:                                                                                                                    
 43 /home/carl/kde6/usr/include/QtCore/qobject.h: In instantiation of ‘T QObject::findChild(QAnyStringView, Qt::FindChildOptions) const [with T = KPIM::SSLLabel*; Qt::FindChildOptions = QFlags<Qt::FindChildOption>]’:                   
 44 /home/carl/kde6/src/libkdepim/src/progresswidget/autotests/statusbarprogresswidgettest.cpp:43:51:   required from here                                                                                                                 
 45 /home/carl/kde6/usr/include/QtCore/qobject.h:158:62: error: static assertion failed: No Q_OBJECT in the class passed to QObject::findChild
 46   158 |         static_assert(QtPrivate::HasQ_OBJECT_Macro<ObjType>::Value,
 47       |                                                              ^~~~~
 48 /home/carl/kde6/usr/include/QtCore/qobject.h:158:62: note: ‘QtPrivate::HasQ_OBJECT_Macro<KPIM::SSLLabel>::Value’ evaluates to false
 49 [36/38] Building CXX object src/progresswidget/autotests/CMakeFiles/progressstatusbarwidgettest.dir/progressstatusbarwidgettest.cpp.o