gdbusactiongroup: Add prototype for g_dbus_action_group_sync()

Even private functions that are actually called across compilation
units should have prototypes.  For g_dbus_action_group_sync(), create
one in gdbusactiongroup-private.h

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gio/gdbusactiongroup-private.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright © 2010 Codethink Limited
+ * Copyright © 2011 Canonical Limited
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation; either version 2 of the licence or (at
+ * your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place, Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Authors: Ryan Lortie <desrt@desrt.ca>
+ */
+
+#ifndef __G_DBUS_ACTION_GROUP_PRIVATE_H__
+#define __G_DBUS_ACTION_GROUP_PRIVATE_H__
+
+#include "gdbusactiongroup.h"
+
+G_BEGIN_DECLS
+
+gboolean
+g_dbus_action_group_sync (GDBusActionGroup  *group,
+                          GCancellable      *cancellable,
+                          GError           **error);
+
+G_END_DECLS
+
+#endif