Use enums from Qt::ItemFlags

It seems that one my compiler we need to use an explicit enum instead of
the constant it represents.
It shouldn't cause any issue as the value is the same.

re/sniffer/sniffermodel.cpp

@@ -137,7 +137,7 @@ QVariant SnifferModel::data(const QModelIndex &index, int role) const
 Qt::ItemFlags SnifferModel::flags(const QModelIndex &index) const
 {
     if (!index.isValid())
-        return 0;
+        return Qt::NoItemFlags;
 
     return QAbstractItemModel::flags(index);
 }