extmod/modbtree: Make FILEVTABLE const to put it in ROM.
@@ -320,7 +320,7 @@ STATIC const mp_obj_type_t btree_type = {
.locals_dict = (void*)&btree_locals_dict,
};
-STATIC FILEVTABLE btree_stream_fvtable = {
+STATIC const FILEVTABLE btree_stream_fvtable = {
mp_stream_posix_read,
mp_stream_posix_write,
mp_stream_posix_lseek,
