[gnome-flashback] backends: fix cast-function-type warning

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 1a523ac6aa6561bb9e3980fb70534a6d1dd693df
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sat Sep 8 23:13:17 2018 +0300

    backends: fix cast-function-type warning

 backends/gf-monitor-config-manager.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/backends/gf-monitor-config-manager.c b/backends/gf-monitor-config-manager.c
index ff196a6..8f69fed 100644
--- a/backends/gf-monitor-config-manager.c
+++ b/backends/gf-monitor-config-manager.c
@@ -62,6 +62,13 @@ struct _GfMonitorConfigManager
 
 G_DEFINE_TYPE (GfMonitorConfigManager, gf_monitor_config_manager, G_TYPE_OBJECT)
 
+static void
+history_unref (gpointer data,
+               gpointer user_data)
+{
+  g_object_unref (data);
+}
+
 static GfMonitor *
 find_monitor_with_highest_preferred_resolution (GfMonitorManager *monitor_manager,
                                                 MonitorMatchRule  match_rule)
@@ -1039,7 +1046,7 @@ gf_monitor_config_manager_get_previous (GfMonitorConfigManager *config_manager)
 void
 gf_monitor_config_manager_clear_history (GfMonitorConfigManager *config_manager)
 {
-  g_queue_foreach (&config_manager->config_history, (GFunc) g_object_unref, NULL);
+  g_queue_foreach (&config_manager->config_history, history_unref, NULL);
   g_queue_clear (&config_manager->config_history);
 }