[gtkmm] gtk/gtkmm/private/object_p.h: Decorate Gtk::Object_Class with GTKMM_API

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

commit 0c88b3ba225b23a070b47916ff501a84ad4cc00a
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri Aug 28 16:10:49 2020 +0800

    gtk/gtkmm/private/object_p.h: Decorate Gtk::Object_Class with GTKMM_API
    
    This way, we can fix gtksourceviewmm builds on Visual Studio as well

 gtk/gtkmm/private/object_p.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtkmm/private/object_p.h b/gtk/gtkmm/private/object_p.h
index 313598f2..362ae89d 100644
--- a/gtk/gtkmm/private/object_p.h
+++ b/gtk/gtkmm/private/object_p.h
@@ -9,7 +9,7 @@
 namespace Gtk
 {
 
-class Object_Class : public Glib::Class
+class GTKMM_API Object_Class : public Glib::Class
 {
 public:
 #ifndef DOXYGEN_SHOULD_SKIP_THIS