py: Fix configurability of builtin slice.
diff --git a/py/sequence.c b/py/sequence.c
index c940d9f..c73f3ba 100644
--- a/py/sequence.c
+++ b/py/sequence.c
@@ -51,6 +51,8 @@
     }
 }
 
+#if MICROPY_PY_BUILTINS_SLICE
+
 bool mp_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, mp_bound_slice_t *indexes) {
     mp_obj_t ostart, ostop, ostep;
     machine_int_t start, stop;
@@ -102,6 +104,8 @@
     return true;
 }
 
+#endif
+
 mp_obj_t mp_seq_extract_slice(uint len, const mp_obj_t *seq, mp_bound_slice_t *indexes) {
     machine_int_t start = indexes->start, stop = indexes->stop;
     machine_int_t step = indexes->step;