[glibmm/wip/dboles/ustring_format_fold_and_test] ustring: Make 1 Stringify member const like others

[Daniel Boles <dboles src gnome org>]

commit f6e3298e4e51d1e3ca4c4847cc26070847a7d2a6
Author: Daniel Boles <dboles src gnome org>
Date:   Sun Oct 14 14:16:20 2018 +0100

    ustring: Make 1 Stringify member const like others
    
    This was the only specialisation of Stringify in which the member
    string_ was not already const. Make it so.

 glib/glibmm/ustring.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/glib/glibmm/ustring.h b/glib/glibmm/ustring.h
index 4135b9a3..145e2d52 100644
--- a/glib/glibmm/ustring.h
+++ b/glib/glibmm/ustring.h
@@ -1069,7 +1069,7 @@ template <class T>
 class ustring::Stringify
 {
 private:
-  ustring string_;
+  const ustring string_;
 
 public:
   explicit inline Stringify(const T& arg) : string_(ustring::format(arg)) {}