ash: fix warning in debug build

diff --git a/shell/ash.c b/shell/ash.c
index fe8c1be..4acc301 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -11848,7 +11848,9 @@
 #endif
 		}
 		n = parsecmd(inter);
-		/* showtree(n); DEBUG */
+#if DEBUG
+		showtree(n);
+#endif
 		if (n == NEOF) {
 			if (!top || numeof >= 50)
 				break;