patch 9.0.2147: Type check tests fail without the channel feature

Problem:  Type check tests fail without the channel feature
Solution: only run tests, when Vim was build with +channel

fixes: #13617
closes: #13619

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/option.c b/src/option.c
index 5727885..98bac5a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3930,7 +3930,7 @@
  * Process the updated 'previewwindow' option value.
  */
     char *
-did_set_previewwindow(optset_T *args)
+did_set_previewwindow(optset_T *args UNUSED)
 {
     if (!curwin->w_p_pvw)
 	return NULL;