Initialize m_tocEnabled

Othewise we get an unitialized memory use

==38984== Conditional jump or move depends on uninitialised value(s)
==38984==    at 0xE5B0125: Okular::Part::slotNewConfig() (part.cpp:3000)
==38984==    by 0xE59FAF1: Okular::Part::Part(QWidget*, QObject*, QList<QVariant> const&) (part.cpp:595)
==38984==    by 0xE5BB596: QObject* KPluginFactory::createPartInstance<Okular::Part>(QWidget*, QObject*, QList<QVariant> const&) (kpluginfactory.h:526)
==38984==    by 0x5401674: KPluginFactory::create(char const*, QWidget*, QObject*, QList<QVariant> const&, QString const&) (in /usr/lib/libKF5CoreAddons.so.5.70.0)
==38984==    by 0x124543: KParts::ReadWritePart* KPluginFactory::create<KParts::ReadWritePart>(QObject*, QList<QVariant> const&) (kpluginfactory.h:545)
==38984==    by 0x11D126: Shell::Shell(QString const&) (shell.cpp:97)
==38984==    by 0x11B250: Okular::main(QStringList const&, QString const&) (okular_main.cpp:169)
==38984==    by 0x118418: main (main.cpp:82)