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/gapplicationimpl-dbus.c

@@ -24,7 +24,7 @@
 #include "gactiongroup.h"
 #include "gactiongroupexporter.h"
 #include "gremoteactiongroup.h"
-#include "gdbusactiongroup.h"
+#include "gdbusactiongroup-private.h"
 #include "gapplication.h"
 #include "gfile.h"
 #include "gdbusconnection.h"
@@ -37,12 +37,6 @@
 #include "gapplicationcommandline.h"
 #include "gdbusmethodinvocation.h"
 
-G_GNUC_INTERNAL gboolean
-g_dbus_action_group_sync (GDBusActionGroup  *group,
-                          GCancellable      *cancellable,
-                          GError           **error);
-
-
 /* DBus Interface definition {{{1 */
 
 /* For documentation of these interfaces, see