[gtkmm/fix.exports.msvc: 1/2] gtk/src/expression.hg: Mark closure_callback_func()

[Chun-wei Fan <fanchunwei src gnome org>]

commit 58431c4056df9df4089df218c6ffa93faecd6772
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Sat Jan 23 10:10:23 2021 +0800

    gtk/src/expression.hg: Mark closure_callback_func()
    
    ...with GTKMM_API so that it will be exported on Visual Studio-style
    builds.

 gtk/src/expression.hg | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/src/expression.hg b/gtk/src/expression.hg
index 2815f62e..5ce86c56 100644
--- a/gtk/src/expression.hg
+++ b/gtk/src/expression.hg
@@ -343,6 +343,7 @@ void closure_marshal(GClosure* closure,
                      gpointer invocation_hint,
                      gpointer marshal_data);
 
+GTKMM_API
 void closure_callback_func();
 
 template<class TI>