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/esp_newlib.h

@@ -4,11 +4,14 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#ifndef __ESP_NEWLIB_H__
-#define __ESP_NEWLIB_H__
+#pragma once
 
 #include <sys/reent.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*
  * Initialize newlib time functions
  */
@@ -57,4 +60,6 @@ void esp_sync_timekeeping_timers(void);
  */
 void esp_newlib_locks_init(void);
 
-#endif //__ESP_NEWLIB_H__
+#ifdef __cplusplus
+}
+#endif