gobject: Make g_clear_object take a non-volatile GObject **

The implementation is silently discarding this anyway, and
g_object_unref() is using atomic operations. So this should be safe.

Having this here triggers -Wdiscarded-qualifiers when g_clear_pointer()
is fixed to use __typeof__().

gobject/gobject.c

@@ -3378,7 +3378,7 @@ g_object_unref (gpointer _object)
  **/
 #undef g_clear_object
 void
-g_clear_object (volatile GObject **object_ptr)
+g_clear_object (GObject **object_ptr)
 {
   g_clear_pointer (object_ptr, g_object_unref);
 }