extmod/modlwip: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register lwip_slip_stream
instead of using a conditional inside of mp_state_vm_t.

Signed-off-by: David Lechner <david@pybricks.com>

extmod/modlwip.c

@@ -1783,4 +1783,6 @@ MP_REGISTER_MODULE(MP_QSTR_lwip, mp_module_lwip);
 // On LWIP-ports, this is the usocket module (replaces extmod/modusocket.c).
 MP_REGISTER_MODULE(MP_QSTR_usocket, mp_module_lwip);
 
+MP_REGISTER_ROOT_POINTER(mp_obj_t lwip_slip_stream);
+
 #endif // MICROPY_PY_LWIP

py/mpstate.h

@@ -198,10 +198,6 @@ typedef struct _mp_state_vm_t {
 
     // root pointers for extmod
 
-    #if MICROPY_PY_LWIP_SLIP
-    mp_obj_t lwip_slip_stream;
-    #endif
-
     #if MICROPY_VFS
     struct _mp_vfs_mount_t *vfs_cur;
     struct _mp_vfs_mount_t *vfs_mount_table;