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

See issue #699.
diff --git a/py/objcell.c b/py/objcell.c
index b2d4200..9e43d7f 100644
--- a/py/objcell.c
+++ b/py/objcell.c
@@ -43,6 +43,7 @@
 
 #if MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_DETAILED
 STATIC void cell_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in, mp_print_kind_t kind) {
+    (void)kind;
     mp_obj_cell_t *o = o_in;
     print(env, "<cell %p ", o->obj);
     if (o->obj == MP_OBJ_NULL) {