[gtranslator] Proper casting to avoid compiler's warning

[Daniel Mustieles García <dmustieles src gnome org>]

commit 44991d2a202efe6c1c4aaafdf6e8ba5e296503cd
Author: Daniel Mustieles <daniel mustieles gmail com>
Date:   Mon Oct 21 10:33:51 2019 +0200

    Proper casting to avoid compiler's warning

 src/gtr-application.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/gtr-application.c b/src/gtr-application.c
index a339cb69..de899a8c 100644
--- a/src/gtr-application.c
+++ b/src/gtr-application.c
@@ -321,7 +321,7 @@ help_activated (GSimpleAction *action,
 {
   GtrApplication *app = GTR_APPLICATION (user_data);
   GtrApplicationPrivate *priv = gtr_application_get_instance_private (app);
-  gtr_show_help (priv->active_window);
+  gtr_show_help (GTK_WINDOW (priv->active_window));
 }
 
 static void