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/tests/reference.c

@@ -132,7 +132,7 @@ test_clear (void)
 static void
 test_clear_function (void)
 {
-  volatile GObject *o = NULL;
+  GObject *o = NULL;
   GObject *tmp;
 
   (g_clear_object) (&o);