[libpanel] dnd: fix signature of GtkDropTarget::enter signal callback

[Christian Hergert <chergert src gnome org>]

commit f1ec9bca85082229be6ba8c731393939dce3dd04
Author: Christian Hergert <chergert redhat com>
Date:   Mon Sep 26 19:38:21 2022 -0700

    dnd: fix signature of GtkDropTarget::enter signal callback
    
    This requires a return value of the preferred action, which we were not
    updating at the call site.
    
    Fixes an issue where we can't DnD panels.

 src/panel-drop-controls.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/panel-drop-controls.c b/src/panel-drop-controls.c
index e1f9911..a26fec4 100644
--- a/src/panel-drop-controls.c
+++ b/src/panel-drop-controls.c
@@ -250,7 +250,7 @@ on_drop_target_leave_cb (PanelDropControls *self,
   panel_drop_controls_drop_finished (self, FALSE);
 }
 
-static void
+static GdkDragAction
 on_drop_target_enter_cb (PanelDropControls *self,
                          double             x,
                          double             y,
@@ -263,6 +263,8 @@ on_drop_target_enter_cb (PanelDropControls *self,
   self->drop_before_page= NULL;
 
   gtk_widget_queue_allocate (GTK_WIDGET (self));
+
+  return GDK_ACTION_MOVE;
 }
 
 static GtkWidget *