raw_nand: correct static analysis tool warning

Correct the following warning given by sparse tool:
include/drivers/raw_nand.h:158:3: warning:
 symbol '__packed' was not declared. Should it be static?

Change-Id: I03bd9a8aee5cdc5212ce5225be8033f1a6e92bd9
Signed-off-by: Yann Gautier <yann.gautier@st.com>

include/drivers/raw_nand.h

 /*
- * Copyright (c) 2019, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2019-2020, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -7,6 +7,7 @@
 #ifndef DRIVERS_RAW_NAND_H
 #define DRIVERS_RAW_NAND_H
 
+#include <cdefs.h>
 #include <stdint.h>
 
 #include <drivers/nand.h>