feat(storage/vfs): make part of nullfs API private

components/vfs/include/esp_private/nullfs.h

@@ -0,0 +1,24 @@
+/*
+ * SPDX-FileCopyrightText: 2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#pragma once
+
+#include "esp_vfs.h"
+#include "esp_err.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Get VFS structure for /dev/null
+ *
+ * @return VFS structure for /dev/null
+ */
+const esp_vfs_t *esp_vfs_null_get_vfs(void);
+
+#ifdef __cplusplus
+}
+#endif