[glom] report layout: Fix the build

[Murray Cumming <murrayc src gnome org>]

commit c6016abee164108e96f2ee6361e7d08542ca0a25
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Dec 4 16:02:29 2018 +0100

    report layout: Fix the build
    
    Minor const correction.
    This was broken by a cherry-picked commit from the gtkmm-4v4 branch.

 glom/mode_design/report_layout/treestore_report_layout.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/glom/mode_design/report_layout/treestore_report_layout.cc 
b/glom/mode_design/report_layout/treestore_report_layout.cc
index 3b0f5894..910daedf 100644
--- a/glom/mode_design/report_layout/treestore_report_layout.cc
+++ b/glom/mode_design/report_layout/treestore_report_layout.cc
@@ -58,7 +58,7 @@ bool TreeStore_ReportLayout::row_drop_possible_vfunc(const Gtk::TreeModel::Path&
 {
   //Get the Row that is being dragged:
   Gtk::TreeModel::Path path_dragged_row;
-  Glib::RefPtr<const Gtk::TreeModel> tree_model_dragged_row;
+  Glib::RefPtr<Gtk::TreeModel> tree_model_dragged_row;
   Gtk::TreeModel::Path::get_from_selection_data(selection_data, tree_model_dragged_row, path_dragged_row);
 
   if(path_dragged_row == dest)