headers: Fix headers across the project

Any word started with underscore followed by and uppercase letter or a
second underscore is a reserved word according with C99.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
diff --git a/include/net/gptp.h b/include/net/gptp.h
index 065ba00..95dbf3a 100644
--- a/include/net/gptp.h
+++ b/include/net/gptp.h
@@ -10,8 +10,8 @@
  *
  */
 
-#ifndef __GPTP_H
-#define __GPTP_H
+#ifndef ZEPHYR_INCLUDE_NET_GPTP_H_
+#define ZEPHYR_INCLUDE_NET_GPTP_H_
 
 /**
  * @brief generic Precision Time Protocol (gPTP) support
@@ -286,4 +286,4 @@
  * @}
  */
 
-#endif /* __GPTP_H */
+#endif /* ZEPHYR_INCLUDE_NET_GPTP_H_ */