gui/qt: AddonsListModel::flags: fix disabling of addon

In the affected path we would like to toggle so that the addon is in a
state where it is not enabled by removing that flag (if present), we
would however remove all flags of the addon as the right-hand side
expression will always evaluate to 0.

This fixes the issue by proper use of bitwise negate.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

modules/gui/qt/dialogs/plugins.cpp

@@ -908,9 +908,7 @@ Qt::ItemFlags AddonsListModel::flags( const QModelIndex &index ) const
     int i_state = data( index, StateRole ).toInt();
 
     if ( i_state == ADDON_UNINSTALLING || i_state == ADDON_INSTALLING )
-    {
-        i_flags &= !Qt::ItemIsEnabled;
-    }
+        i_flags &= ~Qt::ItemIsEnabled;
 
     i_flags |= Qt::ItemIsEditable;