[libnotificatonmanager] Initialise member variable

Summary:

1408== Conditional jump or move depends on uninitialised value(s)

1408== at 0x1F0D8A6D:

NotificationManager::NotificationGroupCollapsingProxyModel::setLimit(int)
(src/kde/workspace/plasma-workspace/libnotificationmanager/notificationgroupcollapsingproxymodel.cpp:112)

1408== by 0x1F0BA091:

NotificationManager::Notifications::Private::initProxyModels()
(src/kde/workspace/plasma-workspace/libnotificationmanager/notifications.cpp:227)

1408== by 0x1F0BBC75:

NotificationManager::Notifications::setGroupMode(NotificationManager::Notifications::GroupMode)
(src/kde/workspace/plasma-workspace/libnotificationmanager/notifications.cpp:584)

1408== by 0x1F09F7B2:

NotificationManager::Notifications::qt_static_metacall(QObject*,
QMetaObject::Call, int, void**) (moc_notifications.cpp:668)

Practical impact is limited as every user has to call setLimit anyway

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23683