vfs, fatfs: Add file truncate for fatfs

components/newlib/platform_include/sys/unistd.h

@@ -0,0 +1,30 @@
+// Copyright 2018 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+#ifndef _ESP_SYS_UNISTD_H
+#define _ESP_SYS_UNISTD_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include_next <sys/unistd.h>
+
+int     _EXFUN(truncate, (const char *, off_t __length));
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* _SYS_UNISTD_H */