[balsa/gtk4] source-viewer: Build with gtk4

[Peter Bloomfield <peterb src gnome org>]

commit 45b7f665aedf500a1c6c0947c0f2a9df583c70a1
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Fri Oct 9 21:32:05 2020 -0400

    source-viewer: Build with gtk4

 libbalsa/source-viewer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libbalsa/source-viewer.c b/libbalsa/source-viewer.c
index 8873f16f0..834317adf 100644
--- a/libbalsa/source-viewer.c
+++ b/libbalsa/source-viewer.c
@@ -47,7 +47,7 @@ lsv_close_activated(GSimpleAction * action,
                     GVariant      * parameter,
                     gpointer        user_data)
 {
-    gtk_widget_destroy(GTK_WIDGET(user_data));
+    gtk_window_destroy(GTK_WINDOW(user_data));
 }
 
 static void