newlib: Fix header issues with C++ guards and implicit inclusion

* Added C++ sentinels if missed
* Used #pragma once, removed macro's with leading underscores
* Updated copyright checker to allow "BSD-2-Clause-FreeBSD AND
Apache-2.0" for newlib files
* Fixed minor compilation issues/implicit inclusions

components/newlib/platform_include/sys/unistd.h

@@ -4,20 +4,19 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#pragma once
 
-#ifndef _ESP_SYS_UNISTD_H
-#define _ESP_SYS_UNISTD_H
+#include <sys/types.h>
+
+#include_next <sys/unistd.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include_next <sys/unistd.h>
-
 int truncate(const char *, off_t __length);
 int gethostname(char *__name, size_t __len);
 
 #ifdef __cplusplus
 }
 #endif
-#endif /* _SYS_UNISTD_H */