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/irq_offload.h b/include/irq_offload.h
index 17beb14..1a15813 100644
--- a/include/irq_offload.h
+++ b/include/irq_offload.h
@@ -8,8 +8,8 @@
  * @file
  * @brief IRQ Offload interface
  */
-#ifndef _IRQ_OFFLOAD_H_
-#define _IRQ_OFFLOAD_H_
+#ifndef ZEPHYR_INCLUDE_IRQ_OFFLOAD_H_
+#define ZEPHYR_INCLUDE_IRQ_OFFLOAD_H_
 
 #ifdef __cplusplus
 extern "C" {