libgimpconfig: fix leak.

As far as I understand, the created types are never "unregistered" and
will live in the process until it ends. It cannot even have a custom
class_finalize() (looking at g_type_register_static()'s code, a CRITICAL
is emitted if you try to provide a class_finalize() implementation).

So let's just free the allocated pspecs at the end of class_init()
instead.

libgimpconfig/gimpconfig-register.c

@@ -174,6 +174,8 @@ gimp_config_class_init (GObjectClass  *klass,
                        g_type_name (G_TYPE_FROM_INSTANCE (pspec)), pspec->name, type_name);
         }
     }
+
+  g_free (pspecs);
 }
 
 static void