Ensure proper locking in QFileInfoGatherer

Ensure proper locking in QFileInfoGatherer

The mutex is used to protect the QFileSystemWatcher instance created by
QFileInfoGatherer, except when calling getFileInfos(). Add a locker
before using QFileSystemWatcher in this function.

Note: it appears that QFileInfoGatherer is misusing QFileSystemWatcher
by calling it from multiple threads. QFileSystemWatcher is an event
driven class, and as such, not thread-safe. So far, no problems have
been reported related to this, so I've left the code as-is.

Change-Id: Ib1429d9399e37120acf8e8d3b989b83c4ce24203
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>