nb/intel/sandybridge/raminit_common.h: add missing stdint.h include

Types from stdint.h are used in that header file without stdint.h being
included.

Change-Id: I71449dd26162dc8420c206285896ac9a8e4e04d4
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38580
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/northbridge/intel/sandybridge/raminit_common.h

@@ -18,6 +18,8 @@
 #ifndef RAMINIT_COMMON_H
 #define RAMINIT_COMMON_H
 
+#include <stdint.h>
+
 #define BASEFREQ 133
 #define tDLLK 512