soc/ti/am335x/header.c: Add missing include

Use of 'offsetof' needs <commonlib/bsd/helpers.h>.

Change-Id: Ie250b20f464909649b2bd038dbb757d5df637486
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44738
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/ti/am335x/header.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
+#include <commonlib/bsd/helpers.h>
 #include <stdint.h>
 #include <symbols.h>