soc/intel/braswell/romstage/romstage.c: Add missing include

Replace unused <stddef.h> with missing <stdint.h>.

Change-Id: I43b8ba5849de30e2ee253382ef85b17f2d0ae589
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43343
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/braswell/romstage/romstage.c

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
 #include <cbmem.h>
-#include <stddef.h>
+#include <stdint.h>
 #include <arch/io.h>
 #include <device/mmio.h>
 #include <console/console.h>