extmod/modlwip: Change #ifdef to #if for check of MICROPY_PY_LWIP.

Otherwise this code will be included if MICROPY_PY_LWIP is defined to 0.

extmod/modlwip.c

@@ -1500,7 +1500,7 @@ STATIC mp_obj_t lwip_print_pcbs() {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(lwip_print_pcbs_obj, lwip_print_pcbs);
 
-#ifdef MICROPY_PY_LWIP
+#if MICROPY_PY_LWIP
 
 STATIC const mp_rom_map_elem_t mp_module_lwip_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_lwip) },