[glibmm] glibmm/objectbase.h: Mark _gobject_cppinstance_already_deleted()

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

commit 5ee38d7583d23e4d96af0b0f823420d1a26ec853
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Mar 12 00:29:13 2020 +0800

    glibmm/objectbase.h: Mark _gobject_cppinstance_already_deleted()
    
    ...with GLIBMM_API.  This is needed for building gtkmm, with Visual
    Studio.

 glib/glibmm/objectbase.h | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/glib/glibmm/objectbase.h b/glib/glibmm/objectbase.h
index 885e6d4a..da42039b 100644
--- a/glib/glibmm/objectbase.h
+++ b/glib/glibmm/objectbase.h
@@ -294,6 +294,7 @@ ObjectBase::get_property(const Glib::ustring& property_name) const
 
 #endif /* DOXYGEN_SHOULD_SKIP_THIS */
 
+GLIBMM_API
 bool _gobject_cppinstance_already_deleted(GObject* gobject);
 
 } // namespace Glib