[gtkmm/fix.exports.msvc: 1/2] gtk/src/expression.hg: Mark functions for export

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

commit 37269ac95681a944a84b6103573580a8553e5339
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Sat Jan 23 10:10:23 2021 +0800

    gtk/src/expression.hg: Mark functions for export
    
    ...with GTKMM_API so that it will be exported on Visual Studio-style
    builds.  This will fix building the demo program.

 gtk/src/expression.hg | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtk/src/expression.hg b/gtk/src/expression.hg
index 2815f62e..08bd9084 100644
--- a/gtk/src/expression.hg
+++ b/gtk/src/expression.hg
@@ -314,6 +314,7 @@ private:
 namespace Expression_Private
 {
 
+GTKMM_API
 void watch_callback(gpointer data);
 
 template<class T, class... ATs>
@@ -343,6 +344,7 @@ void closure_marshal(GClosure* closure,
                      gpointer invocation_hint,
                      gpointer marshal_data);
 
+GTKMM_API
 void closure_callback_func();
 
 template<class TI>