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

See issue #699.
diff --git a/py/objgenerator.c b/py/objgenerator.c
index a4d22aa..e67824d 100644
--- a/py/objgenerator.c
+++ b/py/objgenerator.c
@@ -95,6 +95,7 @@
 /* generator instance                                                         */
 
 STATIC void gen_instance_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
+    (void)kind;
     mp_obj_gen_instance_t *self = self_in;
     print(env, "<generator object '%s' at %p>", mp_obj_code_get_name(self->code_state.code_info), self_in);
 }