pstree: fix "warning: 'handle_thread' defined but not used"

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/procps/pstree.c b/procps/pstree.c
index 4cd8cb4..16649cf 100644
--- a/procps/pstree.c
+++ b/procps/pstree.c
@@ -339,12 +339,14 @@
 		dump_by_user(walk->child, uid);
 }
 
+#if ENABLE_FEATURE_SHOW_THREADS
 static void handle_thread(const char *comm, pid_t pid, pid_t ppid, uid_t uid)
 {
 	char threadname[COMM_LEN + 2];
 	sprintf(threadname, "{%.*s}", COMM_LEN - 2, comm);
 	add_proc(threadname, pid, ppid, uid/*, 1*/);
 }
+#endif
 
 static void mread_proc(void)
 {