py/makemoduledefs.py: Add a way to register extensible built-in modules.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/builtinhelp.c

@@ -79,6 +79,7 @@ STATIC void mp_help_print_modules(void) {
     mp_obj_t list = mp_obj_new_list(0, NULL);
 
     mp_help_add_from_map(list, &mp_builtin_module_map);
+    mp_help_add_from_map(list, &mp_builtin_extensible_module_map);
 
     #if MICROPY_MODULE_FROZEN
     extern const char mp_frozen_names[];