commonlib/bsd, libpayload: Do not include helpers.h in stddef.h

`stddef.h` should only provide the definitions defined by ISO or Posix.
The included `commonlib/bsd/helpers.h` provide a lot of non standard
definitions that may interfere with definitions from the application.

Change-Id: Ia71edbc3ffe6694ff4b971decf3a41f915264bc8
Signed-off-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70116
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/commonlib/bsd/cbfs_mcache.c

@@ -2,6 +2,7 @@
 
 #include <assert.h>
 #include <commonlib/bsd/cbfs_private.h>
+#include <commonlib/bsd/helpers.h>
 
 /*
  * A CBFS metadata cache is an in memory data structure storing CBFS file headers (= metadata).

src/commonlib/bsd/cbfs_private.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0-or-later */
 
 #include <commonlib/bsd/cbfs_private.h>
+#include <commonlib/bsd/helpers.h>
 #include <assert.h>
 
 static enum cb_err read_next_header(cbfs_dev_t dev, size_t *offset, struct cbfs_file *buffer,