Use MP_DEFINE_CONST_DICT macro to define module dicts.

This is just a clean-up of the code.  Generated code is exactly the
same.
diff --git a/py/modstruct.c b/py/modstruct.c
index 8c0cafd..f29c8c1 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -205,16 +205,7 @@
     { MP_OBJ_NEW_QSTR(MP_QSTR_unpack), (mp_obj_t)&struct_unpack_obj },
 };
 
-STATIC const mp_obj_dict_t mp_module_struct_globals = {
-    .base = {&mp_type_dict},
-    .map = {
-        .all_keys_are_qstrs = 1,
-        .table_is_fixed_array = 1,
-        .used = MP_ARRAY_SIZE(mp_module_struct_globals_table),
-        .alloc = MP_ARRAY_SIZE(mp_module_struct_globals_table),
-        .table = (mp_map_elem_t*)mp_module_struct_globals_table,
-    },
-};
+STATIC MP_DEFINE_CONST_DICT(mp_module_struct_globals, mp_module_struct_globals_table);
 
 const mp_obj_module_t mp_module_struct = {
     .base = { &mp_type_module },