[gtkmm] Gtk::DropDown: Fix an #include

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 46e5b1c8d0d98c5fd9efb6ab8610aa6916d28f4b
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Sun Aug 2 16:18:36 2020 +0200

    Gtk::DropDown: Fix an #include
    
    Not absolutely necessary when building with meson.

 gtk/src/dropdown.ccg | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/src/dropdown.ccg b/gtk/src/dropdown.ccg
index d2d53da3..e80f5b61 100644
--- a/gtk/src/dropdown.ccg
+++ b/gtk/src/dropdown.ccg
@@ -15,7 +15,7 @@
  */
 
 #include <gtk/gtk.h>
-#include <stringlist.h>
+#include <gtkmm/stringlist.h>
 
 namespace Gtk
 {