musl: add missing FTW_CONTINUE macro

This is to avoid build failures like below for musl.

  test-recurse-dir.c:23:24: error: ‘FTW_CONTINUE’ undeclared

Co-authored-by: Yu Watanabe <watanabe.yu+github@gmail.com>

src/include/musl/ftw.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include_next <ftw.h>
+
+#ifndef FTW_CONTINUE
+#define FTW_CONTINUE 0
+#endif