py, unix: Allow to compile with -Wunused-parameter.

See issue #699.
diff --git a/py/sequence.c b/py/sequence.c
index 8019e28..2781534 100644
--- a/py/sequence.c
+++ b/py/sequence.c
@@ -98,6 +98,8 @@
 #endif
 
 mp_obj_t mp_seq_extract_slice(mp_uint_t len, const mp_obj_t *seq, mp_bound_slice_t *indexes) {
+    (void)len; // TODO can we remove len from the arg list?
+
     mp_int_t start = indexes->start, stop = indexes->stop;
     mp_int_t step = indexes->step;