FileIndexer: Fix a deadlock by making the mutex recursive

The following deadlock occurs during the first run of the
nepomuk-fileindexer, if you're on batery

QMutex::lockInternal()
QMutexLocker::QMutexLocker
Nepomuk2::IndexScheduler::suspend
Nepomuk2::EventMonitor::pauseIndexing
QMetaObject::activate(QObject*, QMetaObject
Nepomuk2::IndexScheduler::indexingSuspended
Nepomuk2::IndexScheduler::suspend
Nepomuk2::EventMonitor::pauseIndexing
Nepomuk2::EventMonitor::slotPowerManagementStatusChanged
Nepomuk2::EventMonitor::EventMonitor
Nepomuk2::FileIndexer::FileIndexer

This only happens during first run cause that is when the EventMonitor
is connected to the IndexScheduler


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