[glom] Minor const improvements.

[Murray Cumming <murrayc src gnome org>]

commit 50c5893dbc7f7865a39768aa3143a378940a529a
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Feb 10 16:03:20 2016 +0100

    Minor const improvements.

 glom/libglom/data_structure/layout/layoutgroup.cc |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/glom/libglom/data_structure/layout/layoutgroup.cc 
b/glom/libglom/data_structure/layout/layoutgroup.cc
index 78a8b47..c31cdf1 100644
--- a/glom/libglom/data_structure/layout/layoutgroup.cc
+++ b/glom/libglom/data_structure/layout/layoutgroup.cc
@@ -111,7 +111,7 @@ bool LayoutGroup::has_any_fields() const
 {
   for(const auto& item : m_list_items)
   {
-    auto field_item = std::dynamic_pointer_cast<LayoutItem_Field>(item);
+    const auto field_item = std::dynamic_pointer_cast<const LayoutItem_Field>(item);
     if(field_item)
     {
       return true;
@@ -119,7 +119,7 @@ bool LayoutGroup::has_any_fields() const
     else
     {
       //Recurse into the child groups:
-      auto group_item = std::dynamic_pointer_cast<LayoutGroup>(item);
+      const auto group_item = std::dynamic_pointer_cast<const LayoutGroup>(item);
       if(group_item)
       {
         if(group_item->has_any_fields())